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DETERIORATION AND REPAIR OF 
ABOVE GROUND CONCRETE WATER TANKS 
IN ONTARIO, CANADA 

REPORT TO 
ONTARIO MINISTRY OF THE ENVIRONMENT 



September, 1987 



Colder Associates 

W.M. Slater 

& Associates Inc. 



DETERIORATION AND REPAIR OF ABOVE GROUND 
CONCRETE WATER TANKS IN ONTARIO, CANADA 



AUTHORS 



R. GRIEVE W.M. SLATER 

Colder Associates W.M. Slater & Associates Inc. 



L. ROTHENBURG 

Colder Associates 



(i) 



APPLIED RESEARCH GROUP MEMBERS 



G. Aldworth - MacLaren Engineers 

T.I. Campbell - Queen's University 

R. Grieve - Colder Associates 

K. MacKenzie - Dalhousie Materials 

W. Pery - W. Pery Engineering (Deceased) 

L. Rothenburg - Colder Associates 

W.M. Slater - W.M. Slater & Associates Inc. (Chairman) 

R. Staton - MacLaren Engineers 

R. Crawford - Ministry of the Environment 

P. Rostern- - Ministry of the Environment 

M. Toza - Ministry of the Environment 

O. Wigle - Ministry of the Environment 



REPORTS PREPARED UNDER THE APPLIED RESEARCH PROGRAMME FOR 
THE MINISTRY OF THE ENVIRONMENT 

Evaluation of Waterproof Coatings for Concrete \\ater Tanks. 

Mackenzie, K., (Dalhousie Materials), Slater, W.M., (W.M. Slater & Associates 
Inc.) and McCrenerc, P., (Knox Martin Kretch) (Editing). Preliminary, 1985. 

Freeze Protection for Above Ground Concrete Uater Tanks in Cold Regions. 

Aldworth, C, Staton, R., (MacLaren Engineers), and Slater, W.M., 
(W.M. Slater & Associates Inc.). Preliminary, 1985. 

Temperature Monitoring, Ontario Concrete Water Tanks. 

Crieve, R. (Colder .Associates). May, 1984 and February, 1986. 

Ice Loading in Elevated Water Tanks. 

Campbell, T.I. and Kong, W.L., (Queen's LTnivcrsity). .April, 1986. 



(ii) 



FOREWORD 

This report began as a study of the 
problems associated with the 
deterioration of some of the 53 above 
ground concrete water storage tanks 
built in Ontario during the period 1956 
to 1980. It was initiated and funded by 
the Ministry of the Environment (MOE) 
as part of the concrete water tank 
rehabilitation programme which was 
supervised by it's Project Engineering 
Branch 

Concrete tanks are structurally 
straightforward systems but, being 
exposed to severe environmental 
conditions, appear to suffer a rate of 
deterioration which has greatly reduced 
the expected life of the structures. The 
damage ranges from heavy surface 
spalling and cracking to delamination 
and eventual failure of the structure. 
The study showed that the prime factors 
identified as determining the rate of 
concrete structure deterioration were the 
number of freeze-thaw cycles, 
temperature amplitudes and frequencies, 
concrete permeability, hydrostatic 
pressure, location, the effect of steel 
reinforcement embedments, and internal 
ice formations. 

Since construction defects, as well as 
the prime factors listed above, have a 
dominant affect on the accelerated rate 
of deterioration of concrete water tanks 
in Ontario, remedial solutions such as 
repair of joints and honeycombing, 
applying waterproof coatings, insulation, 
and replacement by steel tanks, were 
proposed. 

The study was limited to addressing 
already established problems existing in 
the 53 pre-1981 structures. Many of the 
concrete tanks had site specific 
problems, so basic research into such 
areas, for example, as the design of 
special concrete mixes to improve new 



concrete tank service lives to, say, fifty 
years was not carried out. 

The objective of the study was to be 
directed towards seeking rehabilitation 
solutions for existing structures in order 
to achieve a life expectancy of at least 
25 more years. The applied research 
programme, therefore, was focussed 
mainly in the direction of repair, rather 
than on basic concrete research. 

In spite of the emphasis in the study to 
seek remedial solutions to the various 
problems associated with the rapid 
deterioration and failure of concrete 
water tanks in Ontario, a study of the 
mechanisms which caused some of the 
observed rapid failures was considered to 
be most important in seeking repair 
solutions. Numerous field observations 
of apparently unique and hitherto 
unreported and undocumented types of 
failure in reinforced and prestressed 
concrete water tanks, required some 
attempt to find a scientific explanation 
for the causes of the problems. 

The mechanisms of concrete dilation and 
delaminations, as well as the effect of 
internal ice, air entrainment, thermal 
differential strains and strain rates, and 
other deterioration factors are described 
in the report, but no laboratory or proof 
tests have been carried out to date. 

Since the various factors and 
mechanisms may act concurrently, and 
have not been described in technical 
literature, it is recommended in the 
report that basic research be carried out 
in these areas in the future to try to 
quantify the deterioration identified. 

A brief section in the report gives the 
principal results of the temperature 
monitoring of three concrete tanks, one 
uninsulated and two insulated, from 
three separate locations in central, 
south-west, and north-west Ontario. 



(iii) 



The main objective of the monitoring 
programme was to measure the thermal 
history of an uninsulated tank, and by 
comparison, measure the effectiveness of 
insulation systems developed as possible 
engineering solutions to the concrete 
tank deterioration problem by reducing 
both the number and temperature 
amplitude characteristics of freeze-thaw 
cycles on the tank walls. The detailed 
graphical data is presented in a separate 
report prepared for the MOE by Colder 
Associates in February 1986, entitled 
"Temperature Monitoring, Ontario 
Concrete Water Tanks". Useful 
information such as the number and 
frequency of freeze-thaw cycles, 
differences between exposed solar and 
shaded quadrants, temperature 
differentials and rate of temperature 
change, temperature external to and 
within the concrete walls, etc. has been 
reported and plotted in that report. 
Full analysis of this data and its general 
aspects is not within the scope of this 
report. 



The report discusses the various methods 
used to repair different types of 
concrete tanks and gives 
recommendations for assessment and 
analysis of repair systems. 

Although corrosion of metals is not a 
serious problem in concrete water tanks, 
some corrosion of tendons and other 
metallic components has occurred. The 
report gives examples of this type of 
deterioration, and the remedial methods 
used in the rehabilitation programme. 

Interim guidelines have been prepared 
for the design and construction of new 
concrete water tanks in Ontario based 
on the experience gained during the 
rehabilitation programme, and from the 
applied research carried out. The 
guidelines recommend that internal 
waterproofing and external insulation be 
used as the primary protection against 
the deterioration of above ground 
concrete water tanks in Ontario. 



The main conclusion of the report is 
that, without adequate protection of 
permeable concrete from direct contact 
with water and elimination of cyclic 
freezing in the tank wall, above ground 
concrete water tanks will continue to 
deteriorate rapidly. 



(iv) 
ACKNOWLEDGEMENTS 



The work described in this report is the result of the efforts of many individuals 
who assisted with various tasks from tank inspection and testing through to review 
and drafting of the report. 

The members of the Applied Research Group wish to acknowledge the help, 
encouragement, and support offered by the following Ministry of the Environment 
personnel: L. Benoit, R.K. Brown, R.G. Crawford, J.C.F. MacDonald (retired), 
W.C. Ramsden, P. Rostern, M. Toza, O. Wigle, and T. Wright. 

The authors and the Ministry also wish to thank persons from the Ministry of 
Transportation and Communications, Ontario Hydro and from various consulting 
engineering firms who reviewed draft reports, and offered their advice. In addition 
they wish to thank and acknowledge Wyllie and Ufnal and Knox Martin Kretch for 
the use of some of their inspection and photographic data. 

A special thanks to the typing, drafting and office staff of Golder Associates. In 
particular Philomena D'Souza, Mike Wright, Jim Alexander, and Vic Milligan who 
greatly assisted in the production of this report. 



(V) 



TABLE OF CONTENTS 



1.0 INTRODUCTION 

1.1 General 

1.2 Description 

1.3 Applied Research 

1.4 Rehabilitation Programme 

1.5 Above Ground Water Tank Types 

1.6 Performance Rating 

2.0 DETERIORATION OF ABOVE GROUND CONCRETE WATER TANKS 2- 

2.1 General 2 - 

2.2 Observations 2 - 

2.2.1 External Wall Delamination 2 - 

.1 Post-Tensioned Unbonded (PTU) 2 - 

.2 Post-Tensioned Bonded (PTB) 2 - 

.3 Post-Tensioned Wire Wound Gunite 

Protected (G) 2-1 

.4 Reinforced Concrete Standpipes (RC) 2-2 

.5 Conclusions 2-5 

2.2.2 Internal Wall Delamination 2-5 

2.2.3 Localized Spalling 2-6 

2.2.4 Jack-Rod Spalls 2-7 

2.2.5 Cover Coat Shrinkage and Cracking 2-8 

2.2.6 Quality of Concrete in Water Tanks 2-9 

2.2.7 Summary of Observations 2 -10 

3.0 TEMPERATURE MONITORING 3-1 

3.1 General 3-1 

3.2 Temperature Instrumentation 3-1 

3.3 Test Results 3-2 

3.4 Freezing in Concrete 3-4 

3.5 Seasonal Wall Conditions 3-4 

3.5.1 Thermal Conductivities 3-5 

3.5.2 Autumn - Winter Condition 3-5 

3.5.3 Winter - Spring Condition 3-5 

3.5.4 Insulated Tank Winter Profile 3-6 

3.6 Daily Thermal Cycle 3-6 

3.7 Critical Ambient Temperatures 3-7 

4.0 INVESTIGATION OF THE FREEZE-THAW FAILURE 

MECHANISM IN CONCRETE 4-1 

4.1 General 4-1 

4.2 Literature Review 4-2 

4.3 Some Factors Affecting the Freeze-Thaw Durability Of 

Concrete 4-2 

4.3.1 Saturation of Concrete in Water Tanks 4-3 

4.3.2 Hydrostatic Pressure and Evaporation 4-3 



(vi) 



Table of Contents (contd) 



4.3.3 Air Voids 4-4 

4.3.4 Affect of Prestress on Permeability 4-4 

4.4 Action of Freezing Temperature on Concrete 4-5 

4.4.1 Summary 4-9 

4.5 Standard Freeze-Thaw Tests 4-9 

4.6 Rate of Re-Saturating Dilated Concrete 4-10 

4.7 Stresses in Concrete Due to Frost Induced Expansion .... 4-11 

4.7.1 Tensile Stresses in Tank Walls 4 -12 

4.7.2 Model of Tensile Stress Accumulation 4 -14 

4.7.3 Distribution of Stresses 4 -17 

4.7.4 Summary 4 -18 

4.8 Spot Saturation 4-19 

4.8.1 Jack-Rod Spalls 4-20 

4.9 Hydraulic Pressure "Sandwich" 4-21 

4.10 Conclusions and Recommendations 4 -22 

5.0 REPAIR AND REHABILITATION OF CONCRETE WATER TANKS 

IN ONTARIO 5 - 

5.1 General 5 - 

5.2 Design of Repairs 5 - 

5.2.1General 5 - 

5.2.2 Structural Evaluation 5 - 

.1 Loading 5 - 

.2 Analysis 5-3 

5.3 Repair Methods Developed 5-4 

5.4 Tank Repair Methods 5-4 

5.4.1 Condition Surveys 5-4 

5.4.2 Surface Preparation 5-5 

5.4.3 Delaminations and Spalls 5-5 

5.4.4 Crack Repair 5-6 

5.4.5 Waterproofing 5-6 

5.5 Typical Repair Systems for Various Concrete Tank Types . . 5-7 

5.5.1 Concrete Tank Types 5-7 

5.5.2 RC-S Type Tanks 5-7 

5.5.3 G-S Type Tanks 5-9 

5.5.4 PTU-S Type Tanks 5-12 

5.5.5 PTB-S Type Tanks 5-12 

5.5.6 RC-E Type Tanks 5-12 

5.5.7 G-E Type Tanks 5-13 

5.5.8 PTU-E Type Tanks 5-15 

5.5.9 PTB-E Type Tanks 5-16 

5.5.10 G-G Type Tanks 5-17 

5.5.11 RC-G Type Tanks 5-17 

5.6 Quality Assurance and Measurement 5 -18 

6.0 DETERIORATION OF METALS IN CONCRETE WATER TANKS . 6-1 

6.1 Introduction 6-1 

6.1.1 Role of Metals In Concrete Tanks 6-1 

6.1.2 Deterioration Sequence 6-1 



(vii) 
Table of Contents (contd) 



6.1.3 Importance of Construction Process 6-1 

6.1.4 Summary 6-1 

6.2 Corrosion of Steel Wall Reinforcement 6-2 

6.2.1 Need for Reinforcement 6-2 

6.2.2 Types of Reinforcement 6-2 

6.2.3 Detection of Corrosion 6-2 

6.2.4 Protection of Steel by Quality Concrete 6-3 

6.3 Observation and Repairs 6-3 

6.3.1 Reinforced Concrete Tanks (Type RC) 6-3 

6.3.2 Post-tensioned Bonded Tanks (Type PTB) 6-4 

.1 Description 6-4 

.2 Problems 6-4 

.3 Repair 6-5 

6.3.3 Post-tensioned Unbonded Tanks(Type PTU) 6-5 

.1 Description 6-5 

.2 Problems 6-6 

.3 Repair 6-6 

6.3.4 Gunite Protected Tanks (G Type Tanks) 6-7 

.1 Description 6-7 

.2 Problems 6-8 

.3 Repair 6-9 

6.4 Deterioration Of Metal Components 6-1 

6.4.1 Steel Access Tubes in Elevated Tanks 6-1 

.1 Problem 6-1 

.2 Repair 6-1 

6.4.2 Aluminum Ladders in Water 6-1 

.1 Problem 6-1 

.2 Repair 6-1 

6.4.3 Recommendations 6-1 

6.5 Metal Appurtenances on Concrete Tanks 6 -12 

6.5.1 Description 6 -12 

6.5.2 Observations 6 -12 

6.6 Summary and Conclusions 6 -12 

6.7 Recommendations 6-13 

7.0 CONCLUSIONS 7- 

7.1 Introduction 7 - 

7.2 Concrete Deterioration Mechanisms 7 - 

7.2.1 General 7 - 

7.2.2 Internal Ice 7 - 

7.2.3 Freeze-Thaw With Pressurized Water 7 - 

7.2.4 Rate of Deterioration 7-2 

7.2.5 Freezing in Wall Voids 7-2 

7.3 Expansion Joints 7-2 

7.4 Corrosion of Prestressing Steel 7-2 

7.5 Repair Methods 7-2 

7.5.1 General 7-2 

7.5.2 Bonded Waterproofing Coatings 7-2 



(viii) 



Table of Contents (contd) 



7.5.3 Steel Liners 7-3 

7.5.4 Plastic Liners 7-3 

7.5.5 External Post-Tensioning 7-3 

7.6 Freeze Protection 7-4 

7.6.1 General 7-4 

7.6.2 Insulation and Cladding Systems 7-4 

7.6.3 Mixing and Heating Systems 7-4 

7.6.4 Air Gap Heating 7-4 

7.7 Tank Types Not Recommended 7-4 

8.0 RECOMMENDATIONS 8 

8.1 Introduction 8 

8.2 Design & Construction of New Concrete Water Tanks .... 8 

8.2.1 Codes 8 

8.2.2 Interim Guidelines 8 

8.3 Maximum Head 8-2 

8.4 Technology Transfer 8-2 

8.5 Durability 8-2 

8.6 Further Applied Research 8-2 

9.0 REFERENCES 9-1 

10.0 GENERAL REFERENCES 10-1 

11.0 APPENDIX A 

GUIDELINE RECOMMENDATIONS OF MINIMUM REQUIREMENTS 
FOR THE DESIGN AND CONSTRUCTION OF NEW ABOVE GROUND 

CONCRETE WATER TANKS IN ONTARIO II -1 

11.1 Introduction 11-1 

11.2 Scope 11-1 

11.3 References 11-1 

11.4 Design 11-2 

11.4.1 Design Philosophy 11-2 

11.4.2 Tank Design Requirements 11-2 

.1 Insulation 11-2 

.2 Ice prevention 11-2 

.3 Inspection 11-2 

.4 Mixing/Heating 11-2 

.5 Prestressed Construction 11-2 

.6 Prestress Design 11-2 

.7 Concrete Wall Thickness 11-3 

.8 Waterproofing 11-3 

.9 Coatings 11-3 

.10 Liners 11-3 

11.5 Construction 11-3 

11.5.1 Concrete Quality 11-3 

11.5.2 Slip Forming 11-3 

11.5.3 Jump Forming 11-4 



(ix) 



Table of Contents (contd) 



11.5.4 Vertical Waterstops. . . . 

11.5.5 Concrete Joint Preparation 

11.5.6 Concrete Curing 



11.6 Quality Assurance And Tank Performance Testing 

11.6.1 Leakage Testing 

11.6.2 Final Inspection of Structure 

11.6.3 Heat Loss and Ice Prevention 

11.7 Security And Safety 

11.7.1 Security Fence 

11.7.2 Safety 

11.8 Miscellaneous And Appurtenances 



- 4 

- 4 

- 4 

- 4 

- 4 

- 4 

- 4 

- 4 

- 4 

- 5 

- 5 



12.0 APPENDIX B 

DILATION EXPANSION OF SPHERICAL REGION WITHIN 

A LARGER SPHERE 12-1 

13.0 APPENDIX C 

TERMS OF REFERENCE 13-1 



(X) 



LIST OF PHOTOGRAPHS 

Photo 1- 1 Collapsed tank (DUNNVILLE) 1-1 

Photo 1- 2 Tank implosion (SOUTHAMPTON) 1-2 

Photo 1- 3 Detailed view of imploded section (SOUTHAMPTON). ... 1-2 

Photo 2- 1 Typical deterioration at bottom of reinforced concrete 

'standpipe(\VATFORD) 2-2 

Photo 2- 2 External wall delamination (WATFORD) 2-2 

Photo 2- 3 External wall delamination (CAMLACHIE) 2-3 

Photo 2- 4 Fracture at reinforcing steel (CAMLACHIE) 2-3 

Photo 2- 5 Core section showing fracture at external and internal 

steel (CAMLACHIE) 2-3 

Photo 2- 6 Core through tank wall (ARKONA) 2-5 

Photo 2- 7 Close-up of expanded steel concrete interface 

(ARKONA) 2-5 

Photo 2- 8 External delamination in wire wound post-tensioned 

tank (WOODVILLE) 2-5 

Photo 2- 9 Internal delamination (ALVINSTON) 2-6 

Photo 2-10 Localised spalling (BADEN) 2-6 

Photo 2-11 Deterioration caused by construction glove (BADEN) .... 2-6 
Photo 2-12 Damage caused by water seepage at water-stop 

(VAL CARON) 2-6 

Photo 2-13 Coating deterioration and a jack-rod 

spall (WOODVILLE) 2-7 

Photo 2-14 Jack-rod spall at threaded coupling (BADEN) 2-7 

Photo 2-15 Intact spall recovered from bottom of tank 

(WOODVILLE) 2-7 

Photo 2-16 Typical horizontal cracks and leachate stains 

occurring at a prestressed standpipe (WOODVILLE) .... 2-8 
Photo 2-17 Typical cracks and leachate stains occurring at a 

prestressed ground tank (VAL CARON) 2-8 

Photo 2-18 Deterioration of reinforced concrete standpipe 

(ALVINSTON) 2-9 

Photo 4- 1 Dilation of concrete and subsequent debonding of reinforcing 

steel 4-17 

Photo 5- 1 Internal wall ice at bottom of tank after demolition in 

late spring 5-2 

Photo 5- 2 Surfacing tank wall prior to application of coating 

(HESPELER) 5-6 

Photo 5- 3 Trowelling latex modified mortar (HESPELER) 5-6 

Photo 5- 4 Completed surfacing prior to coating application 

(HESPELER) 5-7 

Photo 5- 5 Installation of post-tensioning anchors. 

(ALVINSTON) 5-8 



(xi) 



List of photographs (cont'd) 



Photo 5- 6 Installation of external post-tensioning tendons 

(ALVINSTON) 5-8 

Photo 5- 7 Typical horizontal cracking of a G-S type tank 

(L'ORIGNAL) 5-9 

Photo 5- 8 Ring support structure for insulation and cladding 

(BADEN) 5-9 

Photo 5- 9 Standpipe prior to repair (BADEN) 5-10 

Photo 5-10 Standpipe after insulation (BADEN) 5-10 

Photo 5-11 Standpipe after insulation (WOODVILLE) 5-10 

Photo 5-12 Standpipe prior to repair (HESPELER) 5-11 

Photo 5-13 Standpipe after insulation (HESPELER) 5-11 

Photo 5-14 Applying coat of MMA to exterior of standpipe 

(BADEN) 5-11 

Photo 5-15 External post tensioning (GLENCOE) 5-12 

Photo 5-16 Steel liner being installed (BRECHIN) 5-12 

Photo 5-17 Tank after rehabilitation including strengthening by 

post-tensioning, installation of new steel liner, 

and insulation and cladding (BRECHIN) 5-13 

Photo 5-18 Internal maintenance inspection of liner paint system 

after one year in service (BRECHIN) 5 -13 

Photo 5-19 Vertical cracking at base of wall due to ineffective 

prestressing.(CHELMSFORD) 5-13 

Photo 5-20 External post-tensioning added to compensate for 

lack of prestressing in wall.(AMHERSTBURG) 5-14 

Photo 5-21 Deteriorated internal thrust ring.(CHELMSFORD) 5-14 

Photo 5-22 Repair of thrust ring (CHELMSFORD) 5-14 

Photo 5-23 Completed repair (CHELMSFORD) 5-14 

Photo 5-24 Tank after leakproofing and before insulation. 

(BRIGDEN) 5-15 

Photo 5-25 Steel support system for insulation and cladding. 

(BRIGDEN) 5-15 

Photo 5-26 Completed rchabilitaion of tank (BRIGDEN) 5-16 

Photo 5-27 Delaminated exterior at source of leak after 

removal of fractured concrete. (CASSELMAN) 5 -16 

Photo 5-28 Delaminated wall.(CASSELMAN) 5-17 

Photo 5-29 Reservoir before repair (PRESTON) 5-17 

Photo 5-30 Reservoir after repair (PRESTON) 5-17 

Photo 5-31 Tank demolition - cutting hole at base.(CALLANDER) ... 5-18 

Photo 5-32 Standpipe toppled (CALLANDER) 5-18 

Photo 5-33 Reinforced concrete tank wall after toppling. 

(CALLANDER) 5-18 

Photo 6- 1 Poorly protected post-tensioning anchorage allowing 

water to enter. (BRIGDEN) 6-6 

Photo 6- 2 Detailed view of strand showing corroded wires. 

(BRIGDEN) 6-6 

Photo 6- 3 Typical longitudinal fracture of one of the wires 

of a strand (BRIGDEN) 6-7 

Photo 6- 4 Start and progression of a typical fracture. 

(BRIGDEN) 6-7 



(xii) 



List of photographs (cont'd) 



Photo 6- 5 Delaminatcd cover coat exposing broken prestressing 

wires (AMHERSTBURG) 6-9 

Photo 6- 6 Corroded prestressing wires under delaminated cover 

coat (CHELMSFORD) 6-9 

Photo 6- 7 Corroded access tube (PRESCOTT) 6-10 

Photo 6- 8 Corroded roof truss (HESPELER) 6-10 

Photo 6- 9 Completed roof repair (HESPELER) 6-10 

Photo 6-10 Aluminium ladder exhibiting severe pitting corrosion 

(BRIGDEN) 6-11 

Photo 6-11 Corroded steel manway cover and bolts (WATFORD) .... 6-12 



(xiii) 
LIST OF FIGURES 

Figure 1.1 Categories of Ontario concrete tanks 1-3 

Figure 2.1 Delamination survey of standpipe (CAMLACHIE) .... 2-4 
Figure 2.2 Jacl<-rod spalls 2-8 

Figure 3.1 Location details of temperature instrumentation (ROCKWOOD) 3-1 

Figure 3.2 1983 autumn data at north side of an uninsulated tank 

(ROCKWOOD) 3-2 

Figure 3.3 1983 autumn data at south side of an uninsulated tank 

(ROCKWOOD) 3-2 

Figure 3.4 1984 winter data on north side of an uninsulated tank 

(ROCKWOOD) 3-2 

Figure 3.5 1984 winter data on the south side of an uninsulated 

tank (ROCKWOOD) 3-2 

Figure 3.6 1984 winter data at an insulated tank in Southern 

Ontario (ALVINSTON) 3-3 

Figure 3.7 1985 winter data at an insulated tank in Northern 

Ontario (EAR FALLS) 3-3 

Figure 3.8 Typical seasonal wall conditions 3-5 

Figure 3.9 Typical thermal profile after insulating 3-6 

Figure 3.10 Daily temperature cycles showing continuous freezing 

at the north side of the tank 3-6 

Figure 3.11 Daily temperature cycle showing daily freeze-thaw 

cycles at the south side of the tank 3-6 

Figure 4.1 Theoretical saturation zone and seepage discharge .... 4-3 

Figure 4.2 Relationship between bleed water paths, water pressure 

flow, and stressing force in water tank walls 4-5 

Figure 4.3 Permeability as a function of confining stress (ref. 16) . . 4-5 

Figure 4.4 Typical experimental length change results from 

thermomechanical measurement (ref. 18) 4-6 

Figure 4.5 Correlation between degree of saturation and resistance 

to freezing and thawing (ref. 14) 4-6 

Figure 4.6 Relationship between temperature and dilation for a 

plain and air entrained concrete at various levels of 

saturation (ref. 19) 4-7 

Figure 4.7 Relationship between irreversible expansion and 

freezing expansion of frost resistant and non-frost 

resistant bricks (ref. 18) 4-7 

Figure 4.8 Maximum dilations v's residual expansions after one 

freeze - thaw cycle for "fully" saturated specimens 

(ref. 22) 4-8 

Figure 4.9 Pore size enlargement after a single freeze - thaw 

cycle 4-8 

Figure 4.10 Schematic diagram of a freeze - thaw cycle 4-9 

Figure 4.11 Typical ASTM freeze-thaw results 4 -10 

Figure 4.12 Effect of concrete quality and water pressure on re- 
saturation time 4-11 

Figure 4.13 Theoretical model of tank stress development 4 -12 

Figure 4.14 Distribution of stress in tank wall 4 -13 



(xiv) 
List of Figures (cont'd) 

Figure 4.15 Permeability and tensile stress for 30m (100 ft.) 

head 4-15 

Figure 4.16 Permeability and tensile stress for 15m (50 ft.) 

head 4-15 

Figure 4.17 Five year accumulated tensile stress related to tank 

diameter for 30m (100 ft. )head 4-16 

Figure 4.18 Five year accumulated tensile stress related to tank 

diameter for 15m (50 ft.) head 4-16 

Figure 4.19 Distribution of stresses in tank wall section under 

increasing linear water pressure 4 -17 

Figure 4.20 Illustration of model used to examine debonding of 

reinforcing steel 4 -18 

Figure 4.21 Distribution of stresses in post-tensioned tank wall 

section 4-18 

Figure 4.22 Formation of jack-rod spalls 4 -19 

Figure 4.23 Schematic development of thawed zone in wall 4 -20 

Figure 4.24 Nature of wall damage at location of unfrozen zone ... 4-21 

Figure 5.1 The 10 concrete tank types in Ontario 5-7 

Figure 6.1 Bonded tendons 

Figure 6.2 Unbonded monostrand 

Figure 6.3 Flaws leading to corrosion of wire wrapped circular 

pipes and tanks 

Figure 12.1 Rupture of a brittle sphere due to dilation 



LIST OF TABLES 



TABLE 1/1 Number of tanks of each type constructed 1-3 

TABLE 1/2 Tank performance rating by category 1-3 

TABLE 2/1 Defect rating 2-1 

TABLE 2/2 Summary of typical water tank core test results 2 -10 

TABLE 4/1 Typical values of permeability of concrete used in dams. 4-5 

TABLE 6/1 Types of steel reinforcement 6-2 



6 - 


4 


6 - 


5 


6 - 


8 


12 


-2 



1.0 



INTRODUCTION 



1.1 General 

In recent years it has become evident 
that concrete water tanks located in 
various regions of Ontario suffer 
distress. 

In 1981 a study was undertaken on 
behalf of the Ministry of the Environ- 
ment by W.M. Slater & Associates Inc. 
to identify the nature and extent of the 
tank deterioration. 

The study revealed (Ref. 1 and 2) that a 
large number of the water tanks 
inspected had deteriorated significantly 
over a short period of time. Although it 
was expected that the life of a water 
tank should be in excess of 50 years, 
most of the tanks studied were less than 
9 years old, with an average age of 
about 6 years. It was noted that two 
water tanks had structurally failed. 




Photo 1-1 Collapsed tank (DUNNVILLE) 

The main conclusion of the study was 
that, in general, the rate of deterior- 
ation was unexpectedly high and, if not 
arrested, would lead to structural failure 



of the tanks. Based on this potentially 
dangerous situation, it was recommended 
that a repair programme be initiated 
immediately. 

1.2 Description 

During the preliminary surveys and 
subsequent remedial work to the tanks 
it was found that little data was 
available on the types of materials used, 
construction records or test results of 
materials. It was, therefore, decided 
that a central system of acquiring data 
and providing transfer of technical 
knowledge gained was essential to the 
successful outcome of the repair 
programme. 

1.3 Applied Research 

Concurrently with the repair programme, 
a programme of applied research was 
initiated in order to provide an under- 
standing of the factors leading to water 
tank deterioration so that current 
remedial measures and the design of 
structures in the future would have the 
advantage of this knowledge. Goldcr 
Associates undertook the examination of 
the physical mechanisms of tank 
deterioration and collection of the 
information obtained from the remedial 
works. Other studies carried out under 
the Applied Research Programme were 
directed towards waterproof coatings, 
freeze protection (both of the concrete 
tank and the stored water), ice loadings, 
and temperature monitoring, 
(see page (i)) 

1.4 Rehabilitation Programme 

An important part of the tank rehabil- 
itation programme in the period 1981 to 
1984 was to record any noted tank 
defects, establish causes where possible. 



I - 1 



obtain samples of defects and subsequent 
repair materials, and record and monitor 
the quality and effectiveness of repair 
methods, materials and applications. 
Prime consultant for the programme was 
W.M. Slater & Associates Inc. Colder 
Associates provided an overview of the 
materials used and specialized testing. 
Design and resident inspection for each 
repair was carried out by various 
consultants who were responsible for the 
supervision and management of the 




Photo 1-2 Tank implosion. 
(SOUTHAMPTON) 

To provide a degree of uniformity in 
inspections, a daily inspection sheet was 
developed by Colder Associates to be 
completed by each resident field 
inspector recording observations on 
ambient conditions, type of work being 
undertaken by the contractor, visual 
estimate of quality, and samples ob- 
tained. As the remedial programme 
proceeded, general patterns of 
deterioration with respect to tank types, 
methods of construction, construction 
defects and the like became evident. 




^4r 




Photo 1-3 Detailed view of imploded 
section. (SOUTHAMPTON) 

It is the purpose of this report to 
illustrate the typical defects occurring in 
concrete water tanks in Ontario and to 
present a mathematical model of the 
mechanisms of deterioration observed 
which are described in sections 4 and 
12. The report summarizes the 
conclusions reached from the applied 
research and proposes reasons for the 
rapid rate of deterioration resulting in a 
service life of under 10 years for many 
concrete water tanks in Ontario. It 
describes the repair methods developed 
for the various defects observed, 
proposes recommendations to improve the 
performance and service life of existing 
tanks, and makes recommendations for 
the design and construction of future 
concrete water tanks in Ontario. 



1.5 Abo>c Ground NVater Tank Types 

Ontario concrete water tanks incorporate 
manv different and indi\ idual design 



features; however, there are essentially 
three different categories, namely: 

• standpipes 

• elevated tanks 

• ground tanks 

Standpipes are cylindrical structures up 
to 46 m (150 ft.) high and 7 to 9 m (25 
to 30 ft.) in diameter. Large storage 
capacity and high internal water 
pressure in the lower portion of 
standpipes are distinct features of this 
category of tanks. (See Figure 1.1). 

Elevated tanks ha\e a smaller storage 
capacity but are capable of providing 
high operating head with relatively low 
internal head, appro.ximately 10 m (30 
ft.). Water pressures on the walls of 
ground and elevated tanks are about half 
the pressure present in standpipes. 

Ground tanks ha\e low operating head, 
about 10 m (30 ft.), but have a large 
capacity due to their diameter, up to 30 
m (100 ft.). These tanks are constructed 
at the ground level and can be classified 



TABLE 1/1 
Number of tanks of each type constructed 




STA.-JDPiFES I SI 



GROUND TANKS (Gl 



Tyce 


Ahsreviaced 


Construction 


Nunber of 


^"•"-*- 


Oesionation 


HetHOG Descripcion 


TanJts 


l 


.c-s 


aeinforceô concrete- 
standpipe 


8 


3 


G-S 


Posc-tensioned wire wound 
gunite protected - 
standpipe 

?ost-tensioned unbonded - 
standoioe 




i 


PTB-S 


Post-tensioned bonded - 
standpipe 




5 


RC-E 


Reinforced concrete - 
elevated 




6 


G-E 


Post-tensioned wire wound 
çunite protected - elevated 




7 


PTU-E 


Post-tensioned unbonded - 
elevated 




8 


PTB-E 


elevated 




' 


G-G 


Post-tensioned wire wound 
gunite protected - ground 




10 


RC-G 


Reinforced concrete - ground 




rc... ' .. 1 



as low head tanks. Within each 
category of above ground tanks built in 
Ontario, there are three main structural 
types as listed in Table 1/1. (see also 
fig. 5.1 page 5-7) 

1.6 Performance Rating 

The performance rating of different 
types of above ground tanks were 
determined in the survey of tanks 
carried out by W.M. Slater & Associates 
Inc. (Ref. 2). Table 1/2 presents the 
average rating of tanks by category 
based on data presented in that report. 

It can be seen that standpipes were 
given the poorest performance rating. 
The performance rating of tanks within 
the same category varies and data can 
be found in Ref. I where the rating 
system developed by W.M. Slater & 
Associates Inc. is described. 

TABLE 1/2 
Tank performance rating by category (1981) 



Figure 1.1 Categories of Ontario 
concrete tanks 





Tank 




Perforr 


ance Ratma i 


Rank 


Category 


N'o. 


(0- 


) Scale» 


1 


Ground 


9 




8.0 


2 

3 


Elevated 

Standpipe 


26 

10 




6.9 
5.9 



sound tank . 



1 - 3 



2.0 



DETERIORATION OF ABO\ E GROUND CONCRETE WATER TANKS 



2.1 



General 



There were 11 main defects revealed in 
the survey of concrete water tanks. 
These are summarized in Ref. 1 and arc 
listed in Table 2/1 below, in order of 
diminishing priority and structural 
importance, but not necessarily cost. 

TABLE 2/1 
DEFECT RATING 



1. Wall delamination. 

2. Vertical cracks in wall. 

3. Wall/floor joint leaks and wall 

deterioration. 

4. Vertical voids in shotcretc. 

5. Spalls caused by jack-rods left in 

walls. 

6. Cover coat delamination and 

debonding from prestressing wires. 

7. Waterproof coating delamination 

and debonding. 

8. Cover coat shrinkage cracking. 

9. Cold joints and horizontal cracks. 

10. Corrosion of prestressing wires. 

11. Corrosion of post-tensioning 

tendons. 



Between 1976 and 1986, approximately 50 
concr.ete water tanks were inspected in 
detail and were partially or fully 
repaired, including the demolition and 
replacement of 1 1 concrete tanks by 
steel tanks. During this process, it was 
found that each tank has its own 
deterioration peculiarities; however, 
several types of defects can be 
associated with individual tank types. 



2.2 



Observations 



2.2.1 External Wall Delamination 

Vertical wall delamination was 
considered to be the most serious 



concrete defect found in concrete water 
tanks, and varied in form according to 
the type of construction used and 
whether the structure was prestressed 
or not. 

E.xamples of dclaminations associated 
with the various types of construction 
are described below: 

.1 Post-Tensioned Unbonded (PTU) 

Two standpipes which failed at 
DUNNVILLE and SOUTHAMPTON, 
exhibited wall delamination on the 
centre lines of tendons which 
apparently filled with water and then 
froze causing splitting of the wall. 
The standpipes were re-built in steel. 

.2 Post-Tensioned Bonded (PTB) 

Investigations of walls at leaks revealed 
serious delamination of the walls at 
CASSELMAN, RED LAKE and PICKLE 
LAKE elevated tanks. The 
dclaminations were on the centre lines 
of the post-tensioning ducts and 
occurred mainly in the bottom of the 
tanks. At PICKLE LAKE, dclaminations 
occurred in the presence of major 
horizontal cracks caused by ice thrust. 
In some cases, the ducts were observed 
to be ungrouted at the delamination 
locations. 

.3 Post-Tensioned Wire Wound Gunite 
Protected (G) 

Many isolated dclaminations were noted 
during repairs to the FENELON FALLS, 
BADEN, and other similar wire wound 
tanks. These external dclaminations 
could also be associated with internal 
jack-rod spalls, a defect discussed in 
greater detail later in this report. At 
VAL CARON, a large diameter wire 
wound post-tensioned tank with a 



gunite cover coat, it w'as found that 
small external delaminations were 
associated with leakage through 
form-ties. 




Photo 2-1 Typical deterioration at 
bottom of reinforced concrete stand pipe 
(WATFORD) 

At AMHERSTBURG and CHELMSFORD 
major external delaminations had 
occurred at locations where there were 
defects in the waterstop between the 
wall and floor of the tanks, as well as 
at other minor locations. 

At WOODVILLE, a post-tensioncd wire 
wound gunite standpipe, a series of 
delaminations occurred around the tank 
at the level of the manhole. Further 
detailed inspection revealed that, due to 
the presence of the manhole, the 
post-tensioning wires had been omitted 
from this region of the tank (sec Photo 
2-8). 



.4 Reinforced Concrete Standpipes 
(RC) 

The areas of delaminations found in 
reinforced concrete standpipes were 
much greater than in the tanks 
constructed of prestressed concrete. 

Photo 2-1 and Photo 2-2 show the 
south face of the WATFORD water 
tower and illustrate typical 
deterioration of the lower section of a 
tank. Delamination occurred at the 
level of the reinforcing steel with little 
or no corrosion of the steel. 

Similar delamination was found during 
repair of the ALVINSTON tank where 
large areas of external and internal 
delamination generally occurred on the 
side of southern exposure. One 
interesting observation from this tank 
was that an internal epoxy resin 
coating had been applied to the lower 
6m (20 ft.) of the tank and all major 
delamination had occurred above this 
level. 

•* >* 




Photo 2-2 External wall delamination 
(WATFORD) 

A third reinforced concrete standpipe 
also exhibited extensive wall 
delamination (CAMLACHIE) and was 
eventually replaced by a new tank. 
Photos 2-3 and 2-4 show the major 
spalled areas. As with the other 
reinforced concrete tanks, no corrosion 
of the reinforcing steel was observed. 



I 

V. 



2 - 2 



M 




Photo 2-3 External wall delaminalion 
(CAMLACHIE) 

Cores taken through the tank walls 
indicated fracture of the concrete at 
both the external and internal 
reinforcing steel (Photo 2-5). 



The CAMLACHIE lank shown in Photo 
2-3 was monitored during the winter of 
1982-83 prior to intended repair. It 
was observed that the majority of 
fracturing occurred during the spring of 
1983, sometimes with explosive force 
accompanied by noise and vibrations of 
the structure. 



Li > t ■ ■ 



^:^. 






I '^Vl 


^^H 




^1 


^^m ^H 


1 ' 


/.' MM 


F 


w-jâR 


''^^^ 



Photo 2-4 Fiaciure at reinforcing steel 
(CAMLACHIE) 



Photo 2-5 Core section showing 
fracture at external and internal steel 
(CAMLACHIE) 



After the detailed tank condition survey 
(Figure 2.1) taken during December 
1982 and after the subsequent damage 
of the 1982-83 winter, a rehabilitation 
design was completed. It was decided, 
however, to replace the tank with a 
steel structure. 

Because of the severity of 
delaminations found in most reinforced 
concrete standpipes and the high cost 
of repairs, strengthening, insulation and 
cladding, the following additional 
standpipes and elevated tanks were, or 
will be demolished and replaced by 
steel tanks - ARKONA, CALLANDER, 
MARKDALE, PITTSBURGH, WEST 
LORNE and WYOMING. 

ARKONA tank, a reinforced concrete 
standpipe was inspected 
internally in summer 1985 after 
dewatering. Delaminations similar to 
the CAMLACHIE and WATFORD tanks 
were found at the bottom of the 
southern exposure. Cores taken in 



2 - 3 




W N 

CLOCKWISE DISTANCE /«OUND INTERIOR OF 
STANDPIPE FROM t MANHOLE 



LEGEND 



A CORE LOCATIONS, DEC.2, 1982. 

T^ BY COLDER ASSOCIATES 
_4- CORE LOCATIONS, NOV. 23, 1982, 

^ BY OTHERS 

LOCATIONS OF HOLLOW AREAS 
DETERMINED BY HAMMER 
SOUNDINGS 



o 



Figure 2.1 Delaminalion survey of slandpipe (CAMLACHIE) 



these regions confirmed that the tank 
wall had delaminatcd at the plane of the 
external steel. In addition the concrete 
surrounding the steel was almost totally 
disintegrated. 

In an effort to obtain a sound core for 
compressive strength testing, the 
ARKONA tank was sounded throughout 
its lower circumference. An area was 
selected at the east exposure just 
outside of a location considered to be 
delaminatcd. Due to the presence of a 
waterstop, the core was extracted in two 
sections. The waterstop was found to 
be defective at that location, however 
both core segments were found to be 



sound. It was noted that the inside 
hoop steel was well bonded to the 
concrete, but the outside hoop steel 
was completely debonded from the 
surrounding concrete and was free to 
move because of considerable 
enlargement of the interface 
surrounding the reinforcing steel. 
Detailed examination of this interface 
revealed that the dcbonding was not 
uniform around the reinforcing steel. 
The concrete interface towards the 
inside of the tank wall mated perfectly 
with the steel but there was a gap of 
about 2 mm surrounding the section of 
reinforcing steel facing the outside 
tank wall. 



2 - 4 



Due to the geometry of the gap the 
more usual eauscs of debonding such as 
lack of cleanliness of the reinforcing 
steel or excessive bleed water during 
concrete placement were considered 
unlikely. Since serious delamination of 
the tank wall had been revealed in other 
cores it was considered possible that the 
condition of this core was a result of 
the deteriorating mechanism and was 
representative of a tank wall shortly 
before delamination (sec Photos 2-6 and 
2-7). 




Photo 2-6 Core through tank wall 
(ARKONA) 



£? 




r>«.'. 4P> "vF* 



Photo 2-7 Close-up of expanded steel 
concrete interface (ARKONA) 

.5 Conclusions 

Based on the above observations it was 
concluded that, in general, where the 
internal water under pressure was 
allowed to penetrate to the external 



surface, external delamination was 
likely; furthermore, reinforced concrete 
standpipcs particularly in the southern 
lower portions of the tank, appeared to 
be more susceptible to external 
delamination than other tank types 
including elevated prestressed concrete 
tanks. In comparing standpipcs, it 
appeared that post-tensioning reduced 
the likelihood of external delamination 
except where internal spalling had 
reduced the wall thickness. It was 
concluded that the physical factors 
involved were the availability of water, 
pressure, and southern exposure, with 
radial compression being a mitigating 
factor. 




Photo 2-8 External delamination in wire 
wound post-tensioned tank 
(WOODVILLE) 

2.2.2 Internal Wall Delamination 

Several different categories of internal 
delamination were observed. The 
categories noted were extensive 
delamination to the depth of the 
internal reinforcing steel, localized 
spalling where the depth of cover to 
the reinforcing steel was shallow, 
internal surface delamination, conical 
spalls associated with jack-rod 
couplings, and at locations adjacent to 
vertical unbonded post-tensioning 
tendons. 

As with external delamination, major 
areas of deep internal delamination 
were invariably associated with 



2 - 5 




Photo 2-9 Internal delaminnlion 
(ALVINSTON) 

reinforced concrete standpipes. Again, 
little or no corrosion of the reinforcing 
steel was noted (Photo 2-9). 

2.2.3 Localized Spalling 

Localized spalling had occurred at many 
tanks and could not be associated with 
any tank type in particular, however, it 
was noted that in the majority of cases 
the localized spalling was associated 
with a shallow depth of cover to the 
reinforcing steel or with individual 
aggregate particles (Photo 2-10). 




Photo 2-10 Localized spalling (BADEN) 

One interesting defect was found at the 
BADEN tank where two construction 
gloves had been accidentally buried in 
the tank wall at separate locations. In 
both cases deterioration was extensive 



and deep around the gloves (Photo 
2-11). In another tank (VAL CARON), 
localized spalling occurred at vertical 
joints and could be associated with 
leakage at the water-stop (see Photo 
2-I2V 




Photo 2-11 Deterioration caused by 
construction glove (BADEN) 

In the majority of tanks, the internal 
coating had deteriorated away from the 
wall (Photo 2-13). Examination of core 
sections revealed that in most cases the 
surface of the concrete was fractured 
to a depth of between 5 mm and 15 mm 
In some cases the deterioration was so 
extensive that the remnants of the 
coating could only be detected by 
microscopic examination. 




Photo 2-12 Damage caused hy water 
seepage at water-stop (VAL CARON) 



2 - 6 



Deterioration of this nature did not 
appear to be associated with tank type 
but seemed to be a function of coating 
type and quality (sec report to Ministry 
of the Environment (1985) titled 
"Evaluation of Waterproof Coatings for 
Concrete Water Tanks" where this 
subject is discussed more fully). 




Photo 2-13 C<;fi// ./ a 

jack-rod spall (WOODl ILLE) 

I.IA Jack-Rod Spalls 

Some tanks had been constructed using a 
slip form method. Hollow pipe rods, 
known as jack-rods, each approximately 
3 m in length are used to raise the slip 
form. As construction proceeds, 
additional lengths are added using 
threaded coupling rods. This results in 
hollow tubes at approximately 3 m 
centres extending the full height of the 
tanks and interrupted at each joint by 
the solid rods. 




In the tanks inspected to date, little 
attempt was made to seal these 
jack-rods; the effectiveness of the seal 
between the roof and the tank walls 
was apparently relied on to prevent the 
ingress of water. Inspection has shown 
that this assumption is untrue and that, 
in most cases, water was able to enter 
the uppermost jack-rod section. This 
water leaks out at the threaded 
coupling, saturating the surrounding 
concrete. 

Observations revealed that these tanks 
experienced internal spalls and that the 
spalls invariably occurred exactly at the 
coupling between the individual rods 
(Photo 2-14). The spalls were always 
conical in shape, and although a small 
amount of totally disintegrated concrete 
was usually present at the apex, the 
spalled concrete was normally intact 
without any sign of disintegration 
(Photo 2-15). In general, as many as 
20 to 30 of these spalls could be 
present in the tanks constructed using 
this system (see Figure 2.2). 




Photo 2-15 Inlact spall recovered from 
bottom of tank (WOODVILLE) 



Photo 2-14 Jack-rod spall ai thrccutcd 
coupling (BADEN) 



2 - 7 



2.2.5 Co>er Coat Shrinkage and 
Cracking 

On tanks where a concrete cover coat 
had been applied to the prcstrcssing 
wires, external delamination was not a 
serious problem, however, the majority 
of these tanks had many cracks 
principally in the horizontal direction, 
(sec Photo 2-16 and Photo 2-17). 




Photo 2-16 Typical horizontal cracks and 
leachate stains occurring at a 
prestressed stand pipe (WOODl'ILLE) 





Figu 



ELEVATION 

re 2.2 Jack-rod spalls 



In most instances these external 
horizontal cracks could be traced 
through to the inside wall. Core 
sections taken through this type of 
defect sometimes indicated that 
freeze-thaw damage was present at a 
microscopic level while in other 
instances the crack had no 
deterioration. The general rule 
observed was that where leaks or 
leachate stains were present on the 
outside, then internally fractured walls 
were likclv. 



Photo 2-17 Typical cracks and leachate 
stains occurring at a prestressed ground 
tank (VAL CARON) 



2 - 8 



2.2.6 Qualit\ of Concrclc in N\atcr 
Tanks 

.As stated in the Portland Cement 
Association's publication, "Design and 
Control of Concrete Mixtures", one of 
the greatest advances in concrete 
technology was the development in the 
mid-1930's of air-entrained concrete. 
The principal reason for using 
intentionally entrained air is to improve 
concrete's resistance to freezing and 
thawing. However, there are additional 
beneficial effects of entrained air in 
both fresh and hardened concrete. 
Known benefits are improved workability, 
resistance to de-icers, and sulphate 
resistance. The latter two factors may 
be explained by the reduction in water 
demand for a given cement content due 
to the improved workability and which 
leads to a reduction in the water cement 
ratio and, hence, produces a less 
permeable cement matrix. 

However, the application of "gunite" is 
not ideal for air entrainment and, as, 
shown in Table 1/1 a number of tanks 
have, in the past, used this method. 

Unfortunately, original quality control 
data obtained during the construction of 
the tanks utilizing this method are not 
readily available; however, it is 
understood that normal quality control 
tests were made and were consistent 
with the Ministry and other relevant 
standards specified at that time. 
Typically, for normal concrete placement 
methods, the requirements were for a 28 
day cylinder strength in excess of 3000 
psi and, where normal concrete 
placement methods were utilized, an 
entrained air content of 5 to 7 percent 
was specified. Table 2/2 gives details of 
typical concrete quality obtained by core 
sampling several of the water tanks 
during remedial works. The results of 
tests for compressive strength indicate 
that, in general, the compressive 
strength of the concrete considerably 
exceeded the required standards of the 



day. As stated above, some concrete 
tanks were constructed using air 
entrained concrete. Other tanks due to 
the concrete application method, were 
not air entrained. Although in most 
cases the air entrained concrete 
contained less air than desirable, 
nevertheless, most of the examples 
given are within the limits of 
"acceptable quality" as judged by the 
Ministry of Transportation and 
Communications Bridge Deck Rehabilita- 
tion manual (Ref. 3). Thus, it can be 
stated that the presence and pattern of 
deterioration was not consistent with 
the lack, or otherwise, of air 
entrainment. Furthermore, it was 
considered that, since most of the 
concrete standpipes exhibited external 
deterioration at the bottom of the 
tanks it would be a remarkable 
coincidence if all poor quality concrete 
was placed in the lower sections of 
these tanks and all durable concrete 
was placed in the uppermost sections. 





^^^^^Kr ^^^^^^^^^H 












VSE'"'^ u ' ^^^^^1 


P^ 


fiÊwi'' ' '' ^^^^^^^H 


É 




ÏÏm 


mi^i 



Photo 2-18 Deterioration of reinforced 
concrete standpipe ( ALVINSTON) 



2 - 9 



TABLE 2/2 



Summary of typical water tank core test results 



Tank Name 


Compressive 


Air 


Spacing 


Remarks 




Strength 


Content 


Factor 






(MPa) 


(%) 


(mm) 




Design 


Tested 


Arkona 


28.0 


45.7 


3.0 


0.281 


Cast Concrete 


Rockwood 


35.0 


84.5 


* 


* 


Shotcrete 




* 


61.6 


* 


* 




Odessa 


35.0 


26.6 


4.1 


0.342 


Cast Concrete 




* 


* 


2.5 


0.214 




Woodville 


32.0 


78.6 


* 


* 


Cast Concrete 




* 


61.4 


* 


* 




Amherstburg 


32.0 


41.4 


1.2 


0.124 


Shotcrete 


Hespeler 


32.0 


88.0 


* 


* 


Shotcrete 



2.2.7 Summary of Obser\ations 

Based on the results of these surveys 
the following generalizations can be 
made with respect to the walls of 

concrete tanks. 

1. All tanks exhibiting general 
external deterioration have an 
ineffective internal coating. 

2. Tanks exhibiting isolated external 
deterioration have an internal defect 
which allows seepage of water toward 
the exterior tank surface. 

3. High pressure tanks exhibit more 
deterioration than low pressure tanks. 

4. Reinforced concrete standpipes 
exhibit the most severe external 
deterioration. 

5. External deterioration tends to be 
oriented towards the south exposure 
(solar quadrant). 

6. There is little galvanic corrosion 
of reinforcing steel in areas of 
exterior and interior wall 
delamination. 



7. Atmospheric corrosion of 
prestressing wires has occurred 
where the concrete cover coat has 
delaminated and separated from the 
wall. 

Apart from the last two points relating 
to corrosion, it can be seen that 
freezing and thawing is the consistent 
underlying phenomenon related to 
deterioration of concrete water tanks in 
Ontario. In some cases, defects which 
may be relatively harmless in other 
structures, significantly alter the 
durability of the tank. In other cases, 
structural types and construction 
methods appear to have a significant 
bearing on the useful life of the tank. 

To help explain these observations, 
published research on the freeze-thaw 
durability of concrete was examined, 
the temperature histories of tanks were 
monitored and a conceptual model 
describing progressive freeze-thaw 
deterioration was developed. These 
subjects are discussed in the following 
sections. 



2 - 10 



3.0 



TEMPERATURE MONITORING 



3.1 General 

Although most of the parameters 
required for this study were available in 
published research, the literature did not 
reveal data of adequate detail to assist 
in providing an understanding of the 
climatic influence on water tanks. 
Consequently in January 1983, as part of 
the on-going applied research, the 
Ministry of the Environment initiated 
the installation of a temperature 
monitoring system at ROCKWOOD, an 
uninsulated concrete water tank. A 
similar system was installed at 
ALVINSTON, the first concrete tank to 
be insulated as part of the tank 
rehabilitation programme. 




Figure 3.1 Location details of 
temperature instrumentation( ROC KWOOD ) 



The temperature monitoring sensors were 
installed in January 1983 prior to 
completion of the ALVINSTON insulation. 
To study the effects of a northern 



environment, a monitoring system was 
installed during a 1984 rehabilitation to 
EAR FALLS, a tank with the first 
combined heating, circulating and 
insulation systems installed for freeze 
protection. 

3.2 Temperature Instrumentation 

The ROCKWOOD temperature monitoring 
system consisted of a total of 55 
temperature sensors and was designed 
such that temperature variations with 
respect to height and solar exposure 
could be analyzed. The assembly 
consisted of four main cables at each 
cardinal point. Each cable had an array 
of sensors at the 20 m (60 ft.), 10 m (30 
ft.), and 3 m (10 ft.) levels. The two 
uppermost arrays consisted of five 
separate sensors embedded into the tank 
wall such that the inside sensor was in 
the water, three sensors were evenly 
spaced inside the tank wall, and one 
sensor was exposed to the atmosphere. 
The lower array (10 ft. level), consisting 
of four sensors, did not have the 
external sensor (see Figure 3.1). 

The system was linked to data logging 
and computer equipment which was set 
to monitor the temperature of the 
sensors at two hour intervals. 

The instrumentation at EAR FALLS and 
ALVINSTON was of similar design, but 
with some of the sensors placed in the 
air gap between the insulation and the 
outside concrete to check actual heat 
losses and accuracy of heat-loss 
calculations (see Figures 3.2, 3.3, 3.4 and 
3.5). 




Figure 3.2 1983 autumn data at north 
side of an uninsulated tank 
(ROCKWOOD) 




Figure 3.3 1983 autumn data at south 
side of an uninsulated tank 
(ROCKWOOD) 

3.3 Test Results 

Figures 3.2 to 3.7 illustrate typical data 
from each of the instrumentation 
systems. 



Figures 3.2 and 3.3 are graphical records 
of typical Autumn data comparing the 
north with the south aspects of the 
ROCKWOOD tank. The water supply is 
obtained from an underground source 
and has a steady inlet temperature of 
approximately 7°C. Figures 3.4 and 3.5 
are graphs of typical data obtained at 
these locations during the winter period. 
Analysis of the data revealed the 
following points. 




Figure 3.4 1984 winter data on north 
side of an uninsulated tank 
(ROCKWOOD) 




Figure 3.5 1984 winter data on the south 
side of an uninsulated tank 
(ROCKWOOD) 

• Temperatures in the North exposure 
are almost continuously below freezing 
throughout January. Freeze-thaw 
cycling occurs almost daily in the South 
exposure during the same period. 

• There is a substantial difference in 
daily temperature fluctuation between 
North and South ambient temperatures. 
This difference is reflected in the 
concrete wall temperature. The 
amplitude of the ambient temperature 
and at 25 mm (1 in), into the wall 
surface at the South exposure is 
approximately 20°C and 15°C, 
respectively. Corresponding amplitudes 
for the North exposure are 7°C and 5°C, 
respectively. These differences in 



3 - 2 



amplitude account for the significant 
difference in the number of occurrences 
of freeze-thaw cycling in the South 
exposure when compared to the North 
exposure. 

• Freezing and thawing regularly takes 
place on the inside of the tank wall and 
has a considerable influence on the 
water temperature close to the tank 
wall. 

• Temperature records of the tank 
water obtained during sub-zero ambient 
conditions indicate that the tank water 
provides very little buffering effect and 
suggests that little water circulation 
takes place inside the tank. Sensors at 
the centre of the water in the tank 
indicate a steady reduction in water 
temperature (throughout the period). 

• Sensors in the water close to the 

tank wall indicate that the water quickly 
freezes at that region of the tank 
forming an ice ring around the wall of 
the tank. It should be noted that for 
the period examined, water close to the 
North wall froze 4 days prior to the 
water at the South wall. 

• The sensor in the water at the 20 m 
(60 ft.) level showed that an ice cap 
forms at the top of the tank to a 
considerable depth (greater than 3 m). 
Visual observations confirmed the 
presence of the ice cap and its presence 
was noted throughout the winter period. 



] 






I •' 



'J il* 



Figures 3.6 and 3.7 are graphical records 
from the insulated ALVINSTON and EAR 
FALLS tanks where, in both cases, water 
is from a surface source. 

Figure 3.6 shows that the insulation has 
a significant moderating influence on the 
air space where minimum temperatures 
are roughly one third of the minimum 
external ambient temperatures. It should 
be noted that the construction of the 

insulation was completed on this tank on 
January 12, 1984. (shown as a vertical 
line in figure 3-6). 




Figure 3.6 1984 winter data at an 
insulated tank in Southern Ontario 
(ALVINSTON) 



Figure 3.7 1985 winter data at an 
insulated tank in Northern Ontario 
(EAR FALLS) 

Figure 3.7 is a graphical record of the 
EAR FALLS tank during January 1985. 
Again, the data illustrates the significant 
moderating influence of the insulation on 
the air gap. It was recorded that the 
temperature of the tank water is largely 
independent and isolated from the 
ambient conditions. 

(Detailed records from which the above 
noted observations were obtained are 
given in an Applied Research Report 
Titled "Temperature Monitoring of 
Ontario Concrete Water Tanks" issued by 
the MOE in April 1986). 



3.4 Freezing in Concrete 



3.5 Seasonal Wall Conditions 



Distilled water exposed to atmospheric 
conditions will freeze at 0°C. The 
freezing point of water, however, is 
lowered with reduction in purity and 
increasing pressure. In concrete 
there is a considerable range of pore 
sizes. Research by Helmuth (Ref. 4) 
demonstrated that supercooling is more 
likely to occur than freezing unless it is 
seeded by a catalyst in a process called 
nucleation. 

The smaller the pore size, the more 
difficult it is for nucleation to occur. 
For this reason, at any point during 
cooling below 0°C, all the water 
occupying cavities smaller than a certain 
size remain unfrozen. The lower the 
temperature the greater becomes the 
quantity of frozen cavities. The 
formation of ice in the pores of 
concrete can occur over a considerable 
temperature range. However, research 
has demonstrated that for practical 
purposes freezing of the pore water 
begins at around -2°C and is essentially 
complete at around -4°C (ref. 5, and 6). 

Another consideration of temperature 
reduction in this range occurs when the 
concrete is saturated. As each cavity 
size is frozen, the passage of water 
through it is prevented. 

The significance of the above phenomena 
is that freezing in the pores of wet 
concrete is not a simple event but 
occurs throughout a range of 
temperatures below 0°C. As stated 
above, it can be considered that the 
majority of freezing is completed at 
-4°C and depending on the degree of 
saturation, is accompanied by a 
reduction in effective permeability. 
When all pores are frozen, therefore, the 
effective permeability of the concrete 
approaches zero starting on the outside 
face of the structure. 



The temperature monitoring of the 
various tanks noted above has revealed 
that significant seasonal wall conditions 
can develop depending on such factors 
as inlet water temperature, the ratio of 
daily water used with respect to tank 
volume, tank exposure conditions and 
geographical location. Where these 
factors are favourable, namely high inlet 
temperatures, high tank turnover and 
sheltered exposure conditions, (e.g. 
ground tank) then ice is less likely to 
form on the inside of the wall. 
However, these conditions are rarely the 
case. For example, at VERNER, an 
elevated tank, a 2 m (6 ft.) ring of ice 
was observed around the perimeter of 
the inside of the wall. At ROCKWOOD, 
an unbonded tendon standpipe, a 1 m (3 
ft.) ring of ice was observed. Divers 
measured the thickness of ice occurring 
during January, 1984, at CASSELMAN, an 
elevated tank, and reported the thick- 
ness of ice to be 300 mm (12 in.) around 
the walls and 150 mm (2 in.) at the 
bottom of the tank. Tests at PORQUIS 
JUNCTION, a reinforced concrete tank, 
revealed that due to a low daily 
requirement compared to volume, the 
effective tank volume was reduced by 
75% by the massive formation of ice 
inside the tank. These observations and 
many others have shown that ice cap, 
wall ice and floor ice, often estimated at 
several hundred tons, regularly form on 
the inside of most tanks during winter 
and are often present until the middle 
of May. 

The formation of ice on the internal 
walls of tanks has several disadvantages 
such as deterioration of the internal 
coating due to physical movement of the 
ice, prevention of internal tank 
inspection for a large portion of the 
year due to the danger of falling ice 
and a reduction in the operating volume 
of the tank. However, the formation of 
ice on the internal wall can also produce 
significant changes in the thermal 



3 - 4 



profile of the tank wall which can ha\c 
serious consequences on its durability 
and is, therefore, worthwhile 
considering in detail. 

3.5.1 Thermal Conducti\ities 

To estimate the thermal profile of the 
various wall conditions the following 
thermal conductivities were used. 



Steel 
43.0 



W/°C-^2 



Concrete (Wet) 

1.81 \\/oç-m2 



• Ice 



2.16 \V/°C- 



• Water 

0.60 W/°C-'"2 

• Air 

0.024 w/oc-""^ 

• Slyrofoam 

0.029 w/°C-"'2 

3.5.2 Autumn - NMnter Condition 

Figure 3.8a is an example of an average 
temperature which regularly occurs 
between autumn and mid-winter when 
the minimum 24 hour ambient 
temperature is -10°C. During this 
period, ice has not yet formed on the 
inside of the wall and therefore the 
tank water maintains the inside of the 
wall above zero (typically 2°C). Thermal 
calculations demonstrate that for this 
condition the maximum depth of total 
freezing (-4°C) is about half the wall 
thickness. 

3.5.3 Winter - Spring Condition 

Figure 3.8b is an example of an average 
temperature profile which can occur 
after the development of a ring of ice 
on the inside surface of the tank and 
typically occurs between mid-winter and 



early spring. In this case, a significant 
thickness of ice has formed on the 
inside surface of the concrete. The 
presence of this ice prevents any 
buffering effect from the internal water 




a) Ice free condition (autumn - winter) 




h) Ice ring present inside the tank 

Figure 3.8 Typical seasonal wall 
conditions 



and, therefore, the entire tank wall can 
be frozen to a point where freeze-thaw 
cycling can occur throughout the 
thickness of the wall. 

The temperature data for January 1984 
obtained from ROCKWOOD illustrates the 
change of temperature profiles as the 
ice ring is formed and is given on 
Figure 3.5. The graph shows that the 



water close to the south wall began to 
freeze around January 17th. Prior to 
this, the temperature of the outside of 
the wall was cycling between -10°C and 
+5°C and the inside face was 
consistently above zero. After January 
17th, the temperature of the water fell 
below zero and ice formed. At that 
point, the entire wall temperature was 
consistently below zero and temperature 
cycles occurring in the wall are clearly 
seen. 




Figure 3.9 Typical thermal profile after 
insulating 



3.5.4 Insulated Tank Winter Profile 

A typical thermal profile through the 
wall of an insulated tank is given in 
Figure 3.9 which illustrates the 
beneficial effect of the insulation when 
the ambient temperature is -10°C. At 
that ambient temperature the air gap 
temperature is approximately -2°C and 
the water in the smaller pores remains 
unfrozen in the outer layer of the 
concrete. Field inspections made during 
winter conditions revealed that internal 
ice formations were essentially 
eliminated, thus improving the thermal 
profile through the tank wall and 
preventing deterioration of the internal 
coating by freeze-thaw cycling on the 
inside wall. In addition it should be 
noted that the insulation reduces the 



amplitude of the daily thermal cycle 
considerably - a significant additional 
benefit. 



3.6 Daily Thermal Cycle 

Figures 3.10 and 3.11 are graphs giving 
details of the thermal history of the 
ROCKWOOD tank between January 14 
and January 17, 1984. They illustrate 
typical thermal cycles occurring 
approximately 50 mm (2 in.) away from 
the outside of the tank wall (ambient) 
and approximately 25 mm (1 in.) into 
the concrete surface and demonstrate 
the substantial thermal difference 
between the North and South exposures. 




Figure 3.10 Daily temperature cycles 
showing continuous freezing at the 
North side of the tank 




Figure 3.11 Daily temperature cycle 
showing daily freeze-thaw cycles at the 
South side of the tank 



3 - 6 



At the North exposure, the maximum 
ambient temperature adjacent to the wall 
tends to be consistently below zero. 
Daily ambient temperature fluctuations 
are about 7°C, with corresponding 
concrete temperature changes of about 
4°C to 5°C, and is similar to shaded 
ambient conditions. At the South 
exposure the temperature fluctuation is 
significantly different. It can be seen 
that the daily amplitude of ambient 
temperature change is about 20°C, with 
a corresponding amplitude of about 15°C 
inside the concrete, from +7°C to -8°C. 

3.7 Critical Ambient Temperatures 

Examination of the detailed temperature 
history of the North and South faces of 
an uninsulated tank has demonstrated 
that the above noted differences are 
consistent throughout the year. During 
all seasons, the South exposure receives 
considerable additional solar energy 
which increases the daily temperature 
amplitude when compared to the North 
exposure. However, the data 
demonstrates that under certain ambient 
temperature conditions, it is this 
additional solar energy received during 
the winter which greatly increases the 
number of freeze-thaw cycles at the 
South exposure. 

Based on the data, the daily amplitude 
on the South face of the wall during 
winter is about 20°C and daily freezing 
and thawing cycles can occur where the 
shaded ambient conditions are between 
-5°C and -15°C and clear sunny 
conditions prevail. 

At the North face of the structure, the 
corresponding daily temperature 



amplitude is about 7°C; consequently, 
the critical range of shaded ambient 
conditions for freeze-thaw cycling to 
occur is between -7°C and -5°C; this 
may occur infrequently. 

The temperature monitoring of an 
uninsulated tank has revealed that, there 
is a difference in the daily temperature 
amplitude between the South and North 
exposures which accounts for the 
difference in the number of freeze-thaw 
cycles experienced by concrete at the 
South exposure. It can be seen 
therefore that the majority of 
freeze-thaw cycles is a result of solar 
radiation and occurs when the 
background ambient temperature is 
within a critical range below zero, and 
solar radiation thaws the concrete. In 
Southern Ontario it appears that the 
critical temperature is between -5°C and 
-15°C at the South exposure and 
between -5°C and -7°C at the North 
exposure. 

Comparing Southern Ontario with other 
regions, it is likely that these basic 
environmental conditions are less 
prevalent in either more Northerly or 
more Southerly climates where the 
shaded ambient conditions are likely to 
be either above or below this critical 
range. Considering these conditions for 
other climates, it is also likely that even 
where ambient conditions are within this 
critical range, reduced solar radiation 
due to the prevalence of cloud cover 
will reduce the amplitude of the daily 
cycle and consequently reduce the 
critical temperature range. This aspect 
of the data may help explain why 
concrete deterioration is so prevalent in 
Southern Ontario. 



3 - 7 



4.0 INVESTIGATION OF THE FREEZE-THAW FAILURE MECHANISM IN CONCRETE 



4.1 



General 



There can be several causes of 
premature deterioration of concrete, 
some chemical in nature or some as a 
result of severe climatic conditions. 
With the exception of the presence of 
chlorides, most chemical agents have to 
be present in significant concentrations 
in order to produce rapid deterioration. 
Since water tanks are being used to 
store drinking water and elaborate 
precautions are made to exclude 
concentrated chemicals, it was inferred 
early in the study that the deterioration 
observed was not chemical in nature. 

Many concrete structures which are 
subjected to extremes of weathering 
exposure will deteriorate unless 
protected. Structures frequently exposed 
to saturation by water, followed by 
cycles of freezing and thawing, 
deteriorate more rapidly than others. 
Such structures include concrete 
pavements, bridge decks, curbs and 
gutters, spillway floors and drainage 
channels. 

Concrete water tanks may incorporate 
some system to prevent water from 
contact with concrete. Coating 
breakdown or an inadequate coating can 
expose the concrete surface to water 
saturation. This can lead to severe 
deterioration of concrete under sub-zero 
temperatures (Ref. 7). The following 
quotation from this source provides an 
insight into the nature of the problem: 

"The effect of freezing on concrete 
tanks constitutes a problem of 
considerable magnitude. Concrete has 
some porosity; and when it is subjected 
to high water heads, the water 
permeates throughout these pore 
structures. It is easy to see the effect 
this will have when the excess water is 



subjected to freezing temperatures. An 
expansion of the water during freezing 
will start a slow breakdown of the 
concrete. The action is known as 
spalling, and unless it is arrested, the 
entire tank may end up as an unusable 
water holding facility. At best, 
expensive repair bills can result." 

The survey of water tanks carried out 
by W.M. Slater & Associates Inc. 
revealed that the protective coating in a 
large number of tanks was ineffective. 
Exposure of concrete to water under 
high hydraulic head combined with 
freezing temperatures undoubtedly 
creates conditions of dangerous exposure. 
Although deterioration of concrete in 
these conditions is a distinct possibility, 
it is far from evident that conventional 
measures such as air entrainment of 
concrete will be able to prevent it. 

While it is technically possible to 
eliminate conditions of severe exposure 
by providing an unconditionally 
impermeable liner, e.g. an internal steel 
shell (Ref. 8) or by providing insulation 
to the tanks (Ref. 8), such drastic 
protective measures add significantly to 
repair or construction costs and should 
be considered only after gaining a 
detailed knowledge of the deterioration 
mechanisms, their rates, and factors 
which affect them. 

It is the purpose of this study to 
explain the mechanisms of concrete 
water tank deterioration and to identify 
factors which affect the deterioration 
rates in order to provide a rational basis 
for evaluation of repair and design 
alternatives. 



4 - 1 



4.2 Literature Review 

The literature on concrete water tanks 
is not extensive and the main emphasis 
is on the structural design of tanks, 
their construction and repair methods 
(Refs. 8, 9, 10, 11, 12). A record of 
19th century water tank failures is 
contained in the monograph "Stand Pipe 
Accidents and Failures" by Prof. W.D. 
Pence who, in 1894, collected data on 
such occurrences from the earliest 
known, to the time of publication. It is 
interesting to note that out of 45 
accidents with steel standpipes presented 
in the book "23 were total wrecks, 14 
were slightly damaged and 8 were 
slightly injured. As far as determined, 
the cause of the accident was in 22 
cases due to water; in 11 cases water 
and ice: 11 were reported due to the 
wind, while a number of cases were from 
failure of foundation". Several more 
cases were reported in Ref. 9 which 
dates back to 1910 and from which the 
above quotation of Prof. Pence's work 
was taken. Since tanks in that era were 
not constructed using concrete, most of 
the accidents described are not directly 
relevant to modern concrete tanks. 
However, the sentiments expressed can 
be associated with current concerns and 
provide echoes from the past. 

The history of modern concrete water 
tanks dates back to 1908, when the first 
U.S. patent on prestressed concrete 
tanks was taken. An interesting history 
of the evolution of modern water tanks 
is described in Ref. 7. This recent 
monograph is apparently one of the few 
publications which clearly states that 
concrete water tanks are problem 
structures in freezing conditions. 

Review of modern literature revealed 
that much of the research on 
freeze-thaw deterioration of concrete 
relates to studies of the material aspects 
of this phenomena. A very significant 
portion of this work is either directly 
related to road structures like pavements 



and bridge decks or is derived from 
observations on the performance of 
these structures. Little research has 
been reported for the freeze-thaw 
durability of concrete under constant 
hydrostatic loading. To our knowledge, 
above ground concrete water tanks have 
not previously been a subject of a study 
concerned with deterioration mechanisms 
under freeze-thaw conditions. 

4.3 Some Factors Affecting the 
Freeze-Thaw Durability Of 
Concrete 

The following remarks were made by 
T.C. Powers in 1966 when introducing 
his paper on "Freezing Effect in 
Concrete". These appropriately state the 
dilemma when considering the action of 
freezing and thawing under hydraulic 
pressure. 

"Specimens of concrete kept continually 
wet on all surfaces by spraying or 
immersion usually become damaged or 
destroyed when they are cooled well 
below the normal freezing point of 
water; if the period of soaking is long, 
nearly all concretes so exposed, air 
entrained or not. cannot withstand 
freezing. On the other hand, concrete 
structures having at least one surface 
exposed to air continually show 
extremely various behaviour, from total 
failure, usually localized, to apparent 
immunity to freezing effects" (Ref. 13). 

Examined from this point of view, the 
concrete tank represents long term 
immersion; however, one face is also 
continually exposed to the air. It 
appears, therefore, that several factors 
are involved in the durability of 
concrete water tanks and not just the 
simple expedient of adding appropriate 
air entrainment. 



4 - 2 



4.3.1 Saturation of Concrete in Neater 
Tanks 

(i) Unsaturated Concrete 

Saturation of concrete can occur in 
natural conditions of weathering 
exposure (Ref. 15). It is normally 
expected, however, that the external 
concrete walls of a structure will not 
become saturated. The major difference 
between water tanks and other types of 
structures is that without an appropriate 
waterproofing system, the applied 
hydraulic pressure will force water 
through the pores of the concrete, and 
result in saturation of the concrete 
walls. 

The rate at which water will flow 
through concrete is a function of its 
porosity which depends on the size, 
distribution and continuity of the pores. 
Research has shown that these factors 
are a function of mix design, hydration, 
curing, construction techniques and the 
like. 

(ii) Saturated Concrete 

Calculations based on D'Arcy's law have 
shown that in the lower portion of a 
typical high pressure head tank, 
saturation may be achieved between 2 
and 4 years and is illustrated in Figure 
4.1. This rate of saturation will of 
course be increased by the presence of 
normal defects such as horizontal and 
vertical cracks. 

It is only after the concrete becomes 
saturated to a critical level, above 90%, 
that the action of freezing and thawing, 
which was previously harmless, starts its 
destructive action. It can be 
demonstrated that even the highest 
quality of concrete will achieve this 
state under the pressure conditions 
prevailing in a typical concrete 
standpipe. 




Figure 4.1 Theoretical saturation zone 
and seepage discharge 

4.3.2 Hydrostatic Pressure and 
Evaporation 

As stated elsewhere, entrained air 
bubbles cannot effectively protect 
saturated concrete from the development 
of freezing and thawing induced pore 
pressures. Where one face of the 
concrete is exposed to air, then, under 
low hydraulic pressure, the evaporation 
process can reduce the level of 
saturation and, therefore, may reduce 
the freeze thaw deterioration. 

For a typical concrete water tank 
structure the factors which determine 
the rate of water entering the pores of 
the concrete are the permeability of an 
internal coating, the presence of cracks 
and fissures in the wall and the pressure 
of the water acting on the wall. This 
last factor indicates that, if the other 
factors are uniform throughout the 
height of the tank, the degree of 
saturation (or perhaps more accurately 
the depth of saturation within the wall) 
increases with hydrostatic pressure and 
is at a maximum at the bottom of the 
tank. Thus, it is likely that the effects 
of evaporation are negated by the 
presence of a sufficiently high 
hydrostatic pressure at the bottom of 
high head tanks. 



4.3.3 Air Voids 

From the point of view of saturation, 
the presence of air voids represents an 
increase in porosity, and therefore, in 
theory, air entrained concrete should 
saturate more readily. However, as 
Verbeck (Ref. 14) points out in his 
discussion on concrete porosity, the 
presence of air voids has other 
beneficial factors such as increased 
workability and reduced segregation, 
which tend to improve the overall 
homogeneity and, consequently, decrease 
the permeability of the concrete 
compared to its non-air entrained 
counterpart. 

The main function of entrained air 
bubbles in the cement paste is to 
prevent the development of internal 
hydraulic pressure. This pressure can be 
produced by the physical increase in the 
volume of ice compared to water. 
However, it can also result from the 
development of osmotic pressure due to 
the presence of alkali in the concrete. 

The water entering the pore space of 
the concrete due to external hydraulic 
pressure will contain a considerable 
quantity of dissolved alkali. As the 
water in the larger pores drops below 
zero some of it freezes. Since it is only 
the water that freezes, the effect is to 
increase the concentration of the alkali 
at that pore space. At other regions, 
the space is smaller and therefore has 
not yet begun to freeze. This water, 
containing dissolved alkali of low 
concentration will tend to move to the 
zone of highly concentrated alkali 
contained in the larger, partially frozen 
pores, creating osmotic pressure. 

The presence of entrained air greatly 
reduces the hydraulic pressures 
generated by either source. In the case 
of hydraulic pressure generated by 
physical expansion of the ice the air 
bubbles act as relieving reservoirs. 
Osmotic pressure is also relieved by the 



air bubbles since any water containing 
dissolved salts and exuding into the 
bubble will immediately freeze. If the 
bubble is not full, then, although the 
dissolved alkali is concentrated by the 
effect of freezing, no osmotic pressure 
can occur due to the discontinuity of 
the fluid. 

Based on the details of these two 
mechanisms, namely, hydraulic pressure 
and osmosis, it can be seen that in 
addition to the importance normally 
given to the quality, size, and spacing of 
the entrained air, it is equally important 
that the air bubbles are not filled or 
even partially filled with moisture since 
the more moisture there is in the air 
bubble the less protection it can impart 
to the cement paste surrounding it. 

As pointed out by T.C. Powers, another 
consideration with respect to air 
entrainment is the fact that some 60% to 
70% of the concrete mass cannot be air 
entrained, namely the aggregate 
particles. Research has indicated that 
concrete, provided it is made with 
saturated aggregates and maintained at a 
high level of saturation prior to test, 
immediately fails a standard freeze-thaw 
test.(Ref. 13, and 17). 

Under high hydrostatic pressure 
conditions, the presence of air voids has 
an effect on the permeability of the 
concrete, however, when these voids are 
filled with water they cannot prevent 
the development of pore pressures during 
freeze-thaw cycles and may aggravate 
the condition. 

4.3.4 Affect of prestress on permeability 

In addition to the factors previously 
described, placement conditions and the 
presence of stress affect the 
permeability (see Figure 4.2). Mills (Ref. 
16) found that the permeability of 
concrete was greater in a direction 
parallel to bleeding and that the 



4 - 4 




Table 4/1 

Typical values of permeability of 

concrete used in dams 



Cûmcnt Lx 


ntent 


Water/cement 


Permeabil 


ity 


kc|/m' 


lb/yd' 


ratio 


10"" m/s 




156 


263 


0.69 


8 




151 


254 


0.74 


24 




138 


235 


0.75 


35 




223 


376 


0.46 


28 





Figure 4.2 Relationship between bleed 
water paths, water pressure flow and 
stressing force in water tank walls. 



FLOW PARALLEL 
TO BLEEDING 



FLOW TRANSVERSE 
TO BLEEDING 



LATERAL STRESS, Cfs , MPo 

Figure 4.3 Permeability as a function of 
confining stress (ref. 16) 

presence of lateral stress reduced 
permeability (see Figure 4.3). In concrete 
water tanks, therefore, water flows in 
the least permeable or horizontal 
direction and circumferential prestressing 
reduces the permeability. Typical values 
of permeability are given in Table 4/1. 
(ref. 19) 



4.4 



Action of Freezing Temperature on 
Concrete 



The durability of concrete structures 
exposed to moisture and freezing 
temperatures has been a major concern 
to engineers for a long period of time. 



Although microscopic mechanisms of 
frost action arc not entirely understood, 
the major features of concrete behaviour 
under freezing temperatures are well 
established. 

• Dry concrete is not affected by frost. 

• Wet, highly saturated concrete 
expands when it freezes. 

The expansion of concrete under 
sub-zero temperatures is frequently 
attributed to an increase in specific 
volume of water during ice formation 
(Ref. 17). Dilation of concrete is then 
attributed to high porewater pressures 
which can be created when excess water 
is forced out of freezing areas causing 
disruption of the internal structure and 
overall expansion of concrete. From the 
point of view of this mechanism, the 
degree of pore saturation is a major 
factor affecting concrete behaviour 
under freezing conditions. 

Brittle porous materials such as 
concrete, rock and bricks all appear to 
exhibit similar behaviour under cyclic 
freezing and thawing in a saturated 
condition. Figure 4.4 is an example of 
laboratory freezing and thawing of clay 
bricks where the dimensional changes 
which can occur in these types of 
materials under one cycle of saturated 
freeze-thaw conditions are shown as a 
solid line. (Ref. 18). 



4 - 5 



As the specimen is cooled to 0°C, 
normal contraction occurs. At a 
temperature of approximately -4°C, the 
specimen undergoes rapid expansion. At 
around -12°C to -15°C, the maximum 
expansion is reached and normal thermal 
contraction is reinstated. During the 
thawing cycle the reverse pattern is 
observed. The specimen expands at a 
slightly greater rate than the rate of 
contraction until it reaches a maximum 
at 0°C. At that point, ice within the 
specimen melts and thawing shrinkage 
occurs. Depending on such factors as 
permeability and pore size, the amount 
of shrinkage is normally less than the 
amount of expansion and consequently 
produces a residual or non-elastic 
expansion after each cycle. 

Also plotted on the right hand axis of 
figure 4.4 is the time history during 
freezing and thawing. The dashed line 
(time v's temperature) shows that the 
entire cycle is completed within 1 hour 
and that both during the freezing cycle, 
and also, during the thawing cycle, a 
delay occurs at -4°C and 0°C 
respectively, indicating that heat is 



0. u 
0. 10 
0.08 
0.06 
04 
0.02 




IfMPERAIURE, 



being absorbed or given off and 
indicates that the water is changing 
state at these temperatures. 

Figure 4.5 illustrates that concrete with 
a low degree of pore saturation is 
immune to cyclic freezing whereas 
similar concretes which have a high 
degree of pore saturation become highly 
susceptible to frost damage. The critical 
saturation level at which dilation occurs 
appears to be about 85% or above. 

Apart from the degree of saturation, the 
magnitude of dilation depends on a 
variety of factors such as strength of 
concrete, permeability of concrete, 
presence of air entrainment and type of 
aggregate. Figure 4.6 illustrates the 
influence of some primary factors on the 
magnitudes of dilation. 



oo-oog- 


-T-1 




-T-i 


n \\ 


> 

































1 










I o 














si^Avo 



60 6S TO 7S 80 as 90 9S 100 

DECREE OF SATURATION, % 



Figure 4.4 Typical experimental length 
change results from thermomechanical 
measurement (ref. 18) 



Figure 4.5 Correlation between degree of 
saturation and resistance to freezing and 
thawing (ref. 14) 



Research has established a relationship 
between the magnitude of residual 
expansion and the total expansion 
occurring during a freezing cycle. 
Figure 4.7 illustrates this relationship 
for bricks and indicates a high degree of 
correlation for both durable and 
non-durable bricks. 



4 - 6 



2 — 



— 


1 

LEVEL OF ^ 


100%/ 


^ 


SATURATION ^ 


\ /se 


■ 


W/C = 0. 60 


1 1 i 




PLAIN MIXES 
111 1 



100% 




W/C : 0.60 

NOMINAL 8% AIR 
ENTRAINED MIXES 

A I \ 



20 15 10 5 -5 -10 -15 -20 20 15 

TEMPERATURE , °C 



10 5 0-5 -10 -15 -20 

TEMPERATURE ,°C 



Figure 4.6 Relationship between temperature and dilation for a plain and air 
entrained concrete at various levels of saturation ( ref. 19 j 



O HOH-FBOST RESISTANT 



- ^fiS^ *^ 






FREEZING EXPANSION, % 



Figure 4.7 Relationship between 
irreversible expansion and freezing 
expansion of frost resistant and non- 
frost resistant bricks (ref. 18) 

Investigations of this nature have also 



Investigations of this nature have also 
been done for concrete. Maclnnis and 
Whiting (Ref. 20) established that a 
similar pattern of dilation occurs during 
freezing of saturated concrete. A total 
of 112 samples cut from concrete paving 
slabs together with laboratory prepared 
specimens were tested. The entrained 
air contents of these samples ranged 
between and 6.5%. The saturation 
procedure was one hour of applied 
vacuum (75 cm mercury), in a dry 
condition; one hour of vacuum in a 
submerged condition, and three days of 
saturation under atmospheric pressure. 
The samples were subsequently 
conditioned into 5 sets representing 
varying degrees of saturation, i.e., 100%, 
88%, 72%, 47% and 30%, and subjected to 
5 freeze- thaw cycles. Length change 
measurements were made during these 
cycles. 



4 - 7 



Figure 4.8 shows a typical length change 
pattern for a fully saturated specimen 
and indicates a similar behaviour pattern 
to that for saturated brick samples; 
dilation commences at about -4°C and on 
completion of the cycle a residual 
expansion remains. Maclnnis reports 
that no dilation occurred on the other 
sets of specimens with a reduced degree 
of saturation. For the saturated 
specimens, the average dilation per cycle 
was 2.2 X 10"* for the air-entrained 
concrete and 1.8 x 10"^ for plain 
concrete. 



Some investigators have carried out 
research on the physical changes that 
occur at a microscopic level during the 
freezing and thawing cycle. Using 
hardened pastes of plaster-of-paris, 
Marusin (Ref. 23) demonstrated that a 
single freeze-thaw cycle was 
accompanied by expansion (dilation), an 
increase of water absorption, a decrease 
in strength and an increase in the 
average pore size as shown on 
Figure 4.9 



This research by Maclnnis corroborates 
other researchers who report that there 
is a critical degree of saturation 
required for dilation to occur. 
Additionally, it suggests that under the 
saturation procedure used the entrained 
air voids may be filled, rendering them 
ineffective against frost action. It is 
argued that this is also likely to occur 
under hydrostatic pressure; this would be 
consistent with the deterioration of 
concrete water tanks in their lower 
section regardless of the presence or 
magnitude of entrained air. 



^1- 




RESIDUAL EXPANSION— X 10 (IN/IN) 

Figure 4.8 Maximum dilations v's residual 
expansions after one freeze - thaw cycle 
for "fully" saturated specimens (ref. 22) 



11 
II 

II 

I 

II 

h 
II 

)! 

// _ 



Figure 4.9 Pore size enlargement after a 
single freeze - thaw cycle 

A schematic diagram of the sequence of 
events is given in Figure 4.10 and 
demonstrates that the residual expansion 
results in a reduction in the degree of 
saturation. If no additional water is 
introduced into the concrete it seems 
reasonable to assume that there will be 
little additional dilation with further 
freeze-thaw cycles since the concrete 
now has a reduced degree of saturation. 
Small additional expansion may occur 
until the concrete is below the critical 
saturation level. However if more water 
is introduced into the concrete, its 
original degree of saturation may be 
achieved and therefore another 
freeze-thaw cycle will produce more 
dilation. From Figure 4.10 it can be 
seen that the volume of water required 
to restore the original level of 
saturation is equal to the volume of 
residual expansion. 



4 - 8 



^ 



Figure 4.10 Schematic diagram of a 
freeze - thaw cycle 

With respect to damage caused by 
freezing and thawing, this concept 
distinguishes between effective and 
non-effective cycles. Some freeze-thaw 
cycles will not be effective in 
contributing to damage if concrete 
previously dilated is not re-saturated 
prior to the onset of the next freezing 
cycle. Although research has 
demonstrated that freezing and thawing 
expansion can occur in one hour (Figure 
4.4) the governing factor in the 
continued effectiveness of freeze-thaw 
cycles to cause damage is the rate of 
water supply during the thaw period. 

4.4.1 Summary 

From the data on the physical changes 
occurring during a single freeze-thaw 
cycle of saturated concrete, it can be 
concluded that 

• Freezing expansion occurs at 
approximately -4°C. 

• Freezing expansion ceases at 
approximately -15°C. 

• Additional expansion occurs during 
the thawing cycle. 

• The entire cycle can occur within a 
period of 1 hour. 



• A residual expansion remains after 
completion of one cycle and is 
directly related to the total 
expansion. 

• There is a critical degree of 
saturation at approximately 90% above 
which significant dilation occurs. 

• A freezing and thawing cycle 
permanently enlarges the size of the 
individual pores. 

4.5 Standard Freeze-Thaw Tests 

To illustrate the failure mechanism 
further, it is worthwhile considering the 
two standard ASTM procedures for 
testing the durability of concrete under 
freezing and thawing cycles, and their 
results. 

In the two Standard ASTM procedures 
(designations C671 and C666) saturated 
concrete is subjected to freeze-thaw 
cycles. 

The major difference between the two 
tests is the frequency of freezing and 
thawing. In the ASTM procedure, C671, 
concrete is kept in water for two weeks 
between cycles, while in procedure C666 
concrete is not allowed to "rest" and 
freeze- thaw cycles follow one another 
within a 2 to 4 hour period. Although 
damage is assessed differently in these 
procedures, in the slow cycle test 
method where specimens are allowed to 
"rest" in water, the concrete is generally 
considered damaged after far fewer 
cycles than those in the rapid method; 
typically the comparative ranges arc 1 to 
10 cycles and 40 to 300 cycles, 
respectively (Figure 4.11a and b). 

The point is that the "rest" in water is 
detrimental because additional water can 
enter the pores at this stage. This is 
precisely the condition of the saturated 
concrete walls of a water tank subjected 
to freezing and thawing. 



4 - 9 




BER OF CYCLES 



a) Relative dynamic modulus (ASTM 
C666) 




NUMBERS OF CYCLES 



b) Dilation - cycle (ASTM C67 1 ) 

Figure 4.11 Typical ASTM frceze-thaw 
results 

The tests described reflect two possible 
types of concrete exposure to frost and 
water: 

• conditions in which the time between 
freeze-thaw cycles is insufficient to 
re-saturate concrete (ASTM designation 
C666 - "Resistance to Rapid Freezing 
and Thawing") 

• conditions in which re-saturation of 
concrete occurs between freeze-thaw 
cycles (ASTM designation C671 - 
"Critical Dilation of Concrete Specimens 
Subjected to Freezing") 



In both cases, concrete can be damaged, 
but in the condition of re-saturation, 
damage occurs in a smaller number of 
cycles. 



The reason for the drastic effect of 
"rest" periods in water is that after 
reducing its degree of saturation in a 
single freezing cycle the concrete then 
absorbs more water and becomes 
re-saturated. The initial loss of 
saturation occurs due to dilation which 
is microscopically translated into an 
increase in average pore size. 

For the next cycle to be as damaging as 
the previous one, this extra volume of 
pores must be filled with water. Two 
weeks "rest" between cycles appears to 
be sufficient to re-saturate normal 
concrete under atmospheric pressure but 
the time is dependent on the coefficient 
of permeability. 

This evidence of the difference between 
standard test methods supports the 
concept that re-saturation of the 
concrete during thawing is the critical 
factor which affects the rate and extent 
of damage due to frost action, 
permeabilities being constant. 

It can be understood that thawing during 
winter conditions will occur more 
frequently in the walls of water tanks 
between the south-east and south-west 
quadrants where the direct rays of the 
winter sun are sufficiently warm to thaw 
the concrete after freezing at night. 
This concept suggests that if 
re-saturation is possible during a thaw, 
then rapid and severe damage could 
occur. It is therefore important to 
examine the time taken to re-saturate 
dilated concrete so that the rate of 
deterioration of concrete tanks can be 
estimated. 

4.6 Rate of Re-Saturating Dilated 
Concrete 

The re-saturation period is the time 
daring which the volume of water 
re-fills that portion of the concrete 
pores equal to the volume increase due 
to the residual expansion. 



10 



To provide a simple model, the rate of 
water influx into concrete pores under a 
given head was assessed using D'Arcy 
flow assumptions. Figure 4.12 illustrates 
re-saturation rates for typical 
permeabilities of good to excellent 
quality concrete (2 to 8 x 10"^'^ m/s) 




built with good quality concrete will not 
re-saturate daily and therefore will be 
less affected by freeze-thaw cycles. 

Damage to concrete during freezing and 
thawing results in general deterioration 
of its mechanical properties i.e. 
reduction of strength and stiffness. 
This degradation is conventionally 
viewed as ultimately leading to spalling, 
frequently observed on the outer layers 
of the affected region. The phenomenon 
of surface spalling, easily detectable 
visually, has masked other mechanical 
and more serious consequences of 
freezing and thawing in concrete water 
tanks in Ontario which cannot be so 
readily observed. 

The next section examines development 
of internal stresses related to 
differential movements created by 
dilation of concrete. 



4.7 



Figure 4.12 Effect of concrete quality 
and water pressure on re-saturation tirrte 



It can be seen that at a pressure head 
of 33 m (100 ft.) excellent quality 
concrete takes approximately 4 days to 
re-saturate but good quality concrete 
can re-saturate within 1 dav. 



Stresses in Concrete Due to Frost 
Induced Expansion 



After each freezing cycle a saturated 
specimen dilates resulting in a residual 
expansion. Continuation of this process 
results in breakdown of the matrix and 
general disintegration of the concrete as 
the pore walls expand and ultimately 
fail. 



This illustrates the reason for the 
divergence in the rate of deterioration 
with respect to concrete quality. 
However, it is clear that concrete with 
permeabilities below the illustrated range 
will deteriorate at the same rate for the 
same pressure head since they can all be 
re-saturated within the period prior to 
the next freezing cycle. Conversely 
concrete with lower permeability will 
deteriorate at a slower rate. It should 
be noted that the permeabilities used for 
this illustration are representative of 
good to excellent quality concrete. 

Another point illustrated in the graph is 
that tanks with a low pressure head and 



In practice however, a concrete member 
can be fully saturated and only frozen 
to a limited depth. In this 
circumstance, part of the concrete 
member is dilating while the remainder 
is unaffected by frost. Under this 
condition, the dilating concrete is 
restrained and will produce reaction 
forces, the magnitude being dependent 
on the geometry of its constraints and 
on the magnitude of unconstrained 
dilation. 



4-11 




DILATING ZONE 




(t-T) 

T 



Û - GAP WHICH WOULD BECREATED WITHOUT 
RESTRAINING ACTION OF INTERNAL SHELL 

P - TENSILE STRESS REQUIRED TO CLOSE GAP 



Figure 4.13 Theoretical model of lank stress development 



In mechanical terms, the resulting 
stresses are of the same nature as 
thermal stresses induced by temperature 
increase. An analogous situation occurs 
when a thick epoxy resin topping is 
applied over a concrete base. On 
heating and cooling, the epoxy resin, 
having a different thermal coefficient 
compared to concrete expands and 
contracts more, resulting in reaction 
forces which break the bond between 
the two layers. Since the magnitude of 
concrete dilation is roughly equivalent to 
a thermal expansion in the range of 
10°C - 30°C, stresses induced by 
freezing and thawing cycles can be 
significant. 

Due to the permanent and irreversible 
nature of the non- elastic volumetric 
changes caused by frost action, stresses 



created in constrained concrete will not 
be relieved immediately after the cycle 
(as is the case in a temperature cycle). 
Some slow relief due to creep may be 
expected, but if freeze-thaw cycles 
follow closely one after another and the 
time between cycles is sufficient to 
permit re-saturation of the concrete, 
stress accumulation will occur eventually 
reaching the rupture limit of concrete. 
Thus, frost related stresses can be much 
more dangerous than temperature related 
stresses. 

4.7.1 Tensile Stresses in Tank Walls 

Stresses in dilating concrete can be 
induced not only by the presence of 
localized external constraints but also 
due to the restraining action of 
different parts of a structure. 



12 



p 
e E 



0020 


T 
R - 


08 




/ i 


06 \ 


0. 1 5 


/ y^ 


~^^\ \ 




1 / 1_ 


-0 04 \ \ 


0.010 


/ / X^"^ 


~~^^\\\ 




I // i 


- 02 \ \ \ 


0.00 5 




^^"^ 



d (X) 




a) Maximum radial stress v's 
position of dilating zone 



b) Distribution of radial stress 
along the wall 



Figure 4.14 Distribution of stress in tank wall 



Consider a situation when tiie outside 
part of a concrete tank wall dilates due 
to freezing, while the internal part of 
the wall is not affected by frost (Figure 
4.13). The dilating outer shell will tend 
to increase in diameter and pull away 
from the non-dilating part of the wall. 
This tendency to differential movement 
will result in tensile stress occurring in 
the entire wall. The tensile stresses 
developed can be calculated as follows: 



radius to wall thickness and depth of 
freezing zone are given in Figure 4.14a 
and indicate the effect of tank radius on 
the development of the induced stresses. 
Figure 4.14b illustrates that the 
maximum stress occurs at the interface 
of the dilating and non-dilating zones 
when this interface is at the centre of 
the wall. In this case, since t = 0.5T, 
tensile stress (P) can be calculated as 
follows: 



eE t (t 



T) 







R T 


where e 


= 


strain due to 
dilatancy 


E 


= 


Young's modulus 
of concrete 


T 


= 


wall thickness 


R 


= 


tank radius 


t 


^ 


thickness of 
dilating concrete 


<^(x) 




radial stress 


p 


= 


c7(.x) maximum 



eE 

4 



Using this simplified formula, with 
typical parameters for good quality 
concrete, stress occurring during one 
freeze- thaw cycle of a saturated 
concrete tank with freezing to half the 
wall thickness is as follows:- 



Calculations for various ratios of tank 
radius to wall thickness and depth of 



4 - 13 



Strain due 

to dilatancy (e) 1 x lO"* 

"*i'oung's, modulus 

of concrete (E) 28 GPa 

(4x10^ psi) 



Tank radius (R) . . 

Wall thickness (T) . 

Average modulus of 
rupture of concrete. 



. 5m (15 ft.) 
. 25 cm (10 in.) 



245 kPa (35 psi) 



Induced tensile stress 

due to single cycle ... 35 kPa (5 psi) 

The apparent modest stress induced in 
one cycle of stress application can 
nevertheless accumulate successively 
under repeated cycles of freezing and 
thawing until the modulus of rupture is 
reached. The mechanism attains a 
maximum rate under the following 
conditions:- 

• re-saturation occurs in the period 
prior to the next freezing cycle; and 



and February of each year. With this 
information, together with published data 
and the previously given formula for 
tensile stress development, a theoretical 
model of stress accumulation was 
constructed. 

4.7.2 Model of Tensile Stress 
Accumulation 

To assess the rate of tensile stress 
build-up under cycles of freezing and 
thawing, a mathematical model 
incorporating the following 
environmental assumptions was 
developed. 

i) Cycles of freezing and thawing follow 
daily for two months every year, i.e. 60 
daily cycles of freezing and thawing per 
annum. 

ii) In each cycle, frost penetrates to the 
middle of the tank wall. 

iii) Concrete is exposed to water under 
pressure continuously applied to the 
inside face of the tank wall. 



• insufficient time is available between 
freezing cycles to permit stress 
relaxations due to creep. 

Compare the laboratory induced 
freeze-thaw cycle (Figure 4.4) with the 
records from the tank monitoring study. 
They are quite similar. It can be seen 
that typically, on a daily basis, the 
temperature of the south side of the 
tank is reduced below -4°C, the 
temperature at which dilation initiates, 
then continues to reduce towards -15°C 
the temperature at which maximum 
freezing dilation occurs. It peaks above 
0°C where thawing and residual 
expansion is accomplished. 

Based on the temperature data from the 
ROCKWOOD experiment, it seems 
reasonable to assume that cycles of 
alternate freezing and thawing would 
occur on a daily basis during January 



iv) Dilation of concrete takes place only 
when concrete becomes re-saturated in 
the period between cycles, i.e. cycles 
occurring in the period when 
re-saturation is in progress are 
"inactive". 

v) Tensile stress relaxation occurs 
continuously. Data on stress relaxation 
in concrete are taken from Ref. 21. 

Computations were performed using 
direct computer simulation of time 
events. The model assumed that the 
tank wall was initially saturated and 
that dilation would occur in the first 
cycle. Using D'Arcy's law, the time 
taken to re-saturate the additional 
volume of pores created by previous 
expansion was calculated. If 
re-saturation was completed in the 
period between two consecutive cycles 
then further dilation was considered to 



4 - 14 



occur causing extra tensile stress in the 
tank wall. 

If the time between cycles was 
insufficient to re-saturate the concrete 
then effective freezing cycles were 
omitted until such time as the water in 
the concrete could be replenished. 




Figure 4.15 Permeability and tensile 
stress for 30m (100 ft.) head 



PRESSURE HEAD 15m 


1501. UNO TANK RADIUS 9m (30fl 


isoo 


PEBMEABILirr 




/v. /\ ...0- m/,.< 




?5!!5:r; 


!/ ' 





Figure 4.16 Permeability and tensile 
stress for 15m (50 ft.) head 

The resultant stress was accumulated for 
a period of 60 cycles. At that point it 
was assumed that no further freezing 
and thawing cycles would occur and the 
accumulated stresses would be gradually 



reduced due to creep for the remaining 
part of that year. This process was 
continued until the concrete had built up 
sufficient tensile stress known to result 
in fracture of the concrete. 

The results of the simulation are given 
in Figures 4.15 and 4.16 for various 
concrete permeabilities at pressure heads 
of 30 m (100 ft.) and 15 m (50 ft.), 
respectively, and illustrate their 
influence over a five year period. In all 
cases the following parameters were 
used: 



strain due to dilation 



1 X 10-* 



wall thickness 25 cm 

(10 in) 



tank diameter. 



. 9 m 
(30 ft.) 



The overall trend of the graphs 
demonstrate the steady accumulation of 
stresses through the winter period and 
their partial relaxation during the rest 
of the year due to creep resulting in a 
"ratchet" effect. Also illustrated is the 
dramatic influence of the internal water 
pressure on stress accumulation, since at 
high pressure heads, practically every 
frceze-thaw cycle produces dilation and 
associated stress. In low internal 
pressure tanks, the stress accumulation 
is not nearly so acute because the 
re-saturation rate is less than the rate 
of freezing and thawing. 

These effects relate to the geometry of 
the tank, however, the graphs also 
illustrate that the permeability of the 
concrete is one of the most important 
governing factors. It should be noted 
that as previously described the 
permeabilities were deliberately selected 
as representing good to excellent quality 
concrete. 

It can be seen that radial tensile stress 
in high pressure standpipes can 
accumulate dangerously over a five year 



4 - 15 



period to a level of tensile stress which 
would fracture concrete. 

It should be noted that due to the 
internal water pressure, this radial 
tensile stress is superimposed on a 
compressive stress of about 140 kPa 
(20psi) which exists in the middle of a 
tank wall. The accumulation of tensile 



PRESSURE HEAD 100 




PET»<EABIUrf 



Figure 4.17 Five year accumulated tensile 
stress related to tank diameter for 30m 
(100 ft.) head. 



'- 




PRESSURE 


HE,0 


50 ft. 










>ooo--^ 


::: 


-^Ti;;^ 


^ 


— ____ 








PERMEABILITY 


— 


— 





— - 


-- 





— 


— 


•'•'O""-'-' 



Figure 4.18 Five year accumulated tensile 
stress related to tank diameter for 15m 
(50 ft.) head. 

stress with time will surpass this 
compressive stress and the net tensile 
stress can reach a level which is 
sufficient to cause the fracture of 
concrete. 



Perhaps one surprising feature of the 
model is the influence of the tank radius 
on the accumulation of stress. For 
example, under a 30m (100 ft.) head of 
water an accumulated stress of 2.1 MPa 
(300 psi) would occur in five years in a 
10 m (30 ft.) diameter tank whereas only 
1.1 MPa (160 psi) tensile stress would 
accumulate under the same head in a 
tank of 20 m (60 ft.) diameter. The 
relationship between tank diameter and 
tensile stress for various permeabilities 
is illustrated in Figures 4.17 and 4.18. 

This particular influence, perhaps masked 
by other factors, has not been noted in 
practice, however, the model 
corroborates many of the observations 
previously mentioned. For example the 
high pressure head of a standpipe results 
in rapid deterioration. Also, 
construction defects which allow the 
water access to the external surface 
effectively amounts to a reduction in the 
wall thickness and hence reduced flow 
path resulting in faster re-saturation. 
Under these circumstances a localized 
section of a low pressure tank could 
behave like a high pressure tank since 
the reduced flow path allows the 
remaining concrete to re-saturate more 
rapidly. Another feature of the model is 
that all tanks will suffer some stress 
accumulation regardless of the quality of 
the concrete because the permeability of 
good quality concrete is insufficient by 
itself to prevent re-saturation, 
particularly under high pressure heads. 
This feature also highlights the 
importance of the internal coating, 
which, if sufficiently impermeable, will 
reduce the re-saturation rates. 

One major difference which has been 
observed in the field is that reinforced 
concrete standpipes deteriorate by 
dclamination more rapidly than 
prestressed concrete standpipes. To 
understand the possible reasons for this 
it is necessary to examine the 
distribution of the stresses as they 
accumulate in the wall of a tank. 



4 - 16 



4.7.3 Distribution of Stresses 

Figure 4.19 illustrates typical forces 
acting on a reinforced concrete wall 
when there is stress accumulation due to 
dilation of saturated concrete. In the 
example given, half the wall thickness is 
subjected to freezing and thawing and is 
restrained by the remaining half of the 
wall section. 



REINFORCED CONCRETE 




Figure 4.19 Distribution of stresses in 
tank wall section under increasing linear 
water pressure. 

As can be seen there is a compressive 
stress acting on the wall due to the 
hydrostatic pressure on the internal face 
of the wall. This pressure is maximum 
at the internal concrete surface and is 
zero at the external surface. Due to 
the restraint provided by the internal 
half of the wall section, reaction forces 
are developed between the dilating and 
non-dilating zones. The distribution of 
stress throughout the wall due to this 
reaction can be calculated as described 
previously. 

These effects represent a general picture 
of stress accumulation and do not 
consider the presence of reinforcing 
steel in the area of tensile stress 
accumulation. The presence of 
reinforcing steel results in local stress 
re-distribution which may eventually lead 
to tensile stress concentrations if the 
bond between the reinforcing steel and 
the concrete is destroyed. 



The example on Figure 4.20 is designed 
to illustrate the possibility of reinforcing 
steel debonding when saturated concrete 
is dilating due to cyclical freezing and 
thawing. Tensile stresses in the 
concrete-steel interface are created by 
the tendency of the dilating material to 
move away from the reinforcement. 
Concrete-steel bond will restrain the 
movement of dilating concrete and 
therefore tensile stresses will develop. 

Calculations show that the interface 
tensile stress induced in one freezing 
cycle is 1.2 MPa (170 psi). This stress 
is based on a dilatant strain of 1.0 x 
10"^. .\ strain twice this value will 
result in debonding of the reinforcement 
in a single freeze-thaw cycle, (see 
appendix B) 

This phenomenon has been observed in 
cores taken in the walls of reinforced 
concrete tanks as shown in Photo 4-1. 
The debonding of the reinforcing steel 
will effectively create cylindrical 
openings in concrete and will induce 
stress concentrations in the matrix 
between the reinforcing steel. 
Depending on the diameter of the steel 
and the spacing between bars, the stress 
concentration can be between 2 to 4 
times the general level of stress. 



>^ 




Photo 4-1 Dilation of concrete and 
subsequent debonding of reinforcing steel 



17 





I-Vc 



ItVc l-t2ii (It Vclll-7Vc) 



: TENSILE STRESS AT THE INTERFACE 

= LINEAR STRAIN DUE TO DILATANCY 

= young's modulus of CONCRETE 

= POISSON'S RATIO OF CONCRETE 



Figure 4.20 Illustration of model used to examine debonding of reinforcing steel 



Consequently, although stress 
accumulation may be greater near the 
centre of the wall, fracture at the level 
of the reinforcing steel is more likely. 



POST TENSIONEO CONCRETE 
TANK'WALL 




Figure 4.21 Distribution of stresses in 
post-tensioned tank wall section 

Figure 4.21 is a diagram of a similar 
situation in a post-tensioned tank. In 
this case, significant radial compressive 
stress caused by post-tensioning offsets 
the magnitude of tensile stresses 
accumulating due to freezing and 
thawing. The post-tensioned tank 
therefore would require considerably 
more freeze-thaw cycles to accumulate 



sufficient net tension to fracture 
concrete at the centre. 

The above theoretical models appear to 
support the field observations that 
reinforced concrete standpipes are more 
susceptible to delamination due to 
freezing and thawing than are 
post-tensioned tanks where the radial 
compression of the post-tensioning wires 
or tendons reduce or eliminate the 
tendency to cause tensile fracture. 
Post-tensioning may also be beneficial 
since it produces a lateral stress which, 
as previously mentioned, reduces the 
permeability of the concrete (Ref. 16). 

4.7.4 Summary 

In the above analyses, concrete tank 
walls were assumed to be uniformly 
saturated and subjected to uniform 
freezing temperatures. This rather 
simplistic set of conditions appears to be 
adequate to explain general trends in the 
deterioration of concrete tanks as 
follows:- 

• Standpipes deteriorate more rapidly 
than elevated or ground tanks since the 
internal pressure in standpipes is 
sufficient to re-saturate concrete on a 
daily basis. 



4 - li 



-RESTRAINT 



DILATING 
SATURATED 
ZONE 



INTERNAL 
SURFACE - 






> u 



-V 



\ 



INTERNAL 
SURFACE- 




FRACTURE INITIATION 
( DILATANT STRAIN ^ 3 x lO"' 
ie 3 CYCLES 



UNSTABLE ACCUMULATED 
FRACTURE GROWTH 

DILATANT STRAIN l^i: lOx lO""* ) 



Figure 4.22 Formation of Jack-rod spalls 



• Post-tcnsioned tanks do not 
externally delaminate to the same extent 
as reinforced concrete standpipcs due to 
the presence of radial compression. 

• Under high internal pressure 
conditions, normal high quality concrete 
may not be sufficiently impermeable to 
eliminate effective freeze-thaw cycles by 
substantially reducing the rate of 
re-saturation. 

• Defects and cracking in tanks can 
reduce the effective wall thickness and 
allow a rate of re-saturation which may 
induce delamination even in low pressure 
tanks. 



4.8 Spot Saturation 

In the previous analyses, concrete tank 
walls were assumed to be uniformly 
saturated and subjected to uniform 
freezing temperatures. This rather 
idealized set of conditions is adequate to 
analyse general trends in tensile stress 
build-up under freezing cycles, but is 
unable to predict random patterns of 
concrete deterioration accompanied by 
spalling. 

In actual service conditions, concrete 
tank walls may not be saturated in all 
areas, due to, for example, the partial 
presence of protective coating, local 
variations in quality, permeability, and 
porosity or non uniform exposure to 



19 



external moisture. In practice, 
therefore, spot saturation is the rule 
rather than the exception. 

The presence of a saturated zone 
dilating under freezing conditions can 
result in a considerable internal stress 
due to the restraining action of the 
non-saturated and, therefore, 
non-dilating area. 

Depending on the geometry of the 
dilating zone and its proximity to the 
concrete surface, dilation can be 
accompanied by tensile stresses resulting 
in spalls. Due to an almost unlimited 
variety of geometrical possibilities and 
the difficulties in estimating the 
resulting stresses, the precise 
mechanisms of spalling cannot be readily 
identified. 

One case of increased restraint that 
merits detailed explanation, is the case 
where a regular pattern of internal 
spalling invariably occurs at the coupled 
joints between jack-rod sections. 

4.8.1 Jack-Rod Spalls 

As previously described, jack-rod spalls 
occur where the hollow rods used for 
slip forming have not been filled with 
grout. Inspection showed that typically 
20 to 30 spalls occurred in each of the 
tanks constructed using this method and 
they invariably occurred at the jack-rod 
junction (see Figure 4.22). 
An attempt has been made to explain 
spalls by ice formation within the hollow 
jack-rods. Although this mechanism 
could account for some observed vertical 
splitting along the length of the tube, it 
could not explain the regular occurrence 
of spalls at the couplings. 

The observed pattern of spalls is 
attributed to saturation of concrete in 
the area of the jack-rod couplings. It 
is reasonable to assume that water would 
leak from the couplings and would 
create a zone of saturated concrete 



which would dilate on freezing. The 
concrete outside the zone would provide 
a restraint to dilation and tensile 
stresses outside of the dilating zone 
would be developed, (see Figure 4.22). 




a) Thermal profile produced by frozen 
internal wall section 




{h)Tliawed zone trapped between two 
frozen zones 

Figure 4.23 Schematic development of 
thawed zone in wall 

Calculations show that a dilating zone of 
about 25 mm radius (1 in.) will initiate 
local fracture when the dilating strain is 



4 - 20 





^ FROZEN :ONÇ 

1 \ TRANSITION ZONE 





Figure 4.24 Nature of wall damage at location of unfrozen zone 



about equal to the limiting tensile strain 

of concrete (3 x 10 ^). Accumulated 

dilatancy of this order of magnitude can 

be reached in several freeze-thaw cycles 

(typically 3 cycles where the dilatancy 

strain is 1 x 10 ^). Subsequent dilation 

cycles will result in fracture propagation 

to the concrete surface. Calculations 

show that this would occur when an 

-4 
accumulated dilatancy of about 10 x 10 

has been achieved in the area near the 



coupling. 

4.9 Hydraulic Pressure "Sandwich" 

Another mechanism can be envisaged 
which could result in the generation of 
considerable hydraulic pressure and, 
therefore, produce the types of 
deterioration witnessed in many of the 
concrete water tanks. This mechanism 
requires the formation of an ice ring 



4 - 21 



inside the tank which allows the 
concrete wall to freeze throughout its 
thickness. As previously discussed, this 
usually occurs during mid-winter to 
spring in the majority of tanks. Since 
this mechanism may well be additive to 
the previously discussed dilation 
mechanism, it may help explain why an 
increase in the extent of deterioration is 
often more noticeable during spring than 
during the autumn. 

As stated in Section 3.4, when saturated 
concrete is frozen, one of the main 
effects is to reduce the effective 
permeability. Subsequent freezing and 
thawing cycles occurring at the exterior 
of the tank wall can push a zone of 
unfrozen water ahead of the freezing 
front which may be accommodated until 
it reaches the frozen layer. At that 
point, due to the reduced permeability, a 
hydraulic pressure may be generated 
which could be sufficient to rupture the 
concrete matrix. This mechanism has 
been duplicated in the laboratory by 
Adkins.'(ref. 25) 

Figures 4.23a and b, are sketches 
representing thermal conditions which 
can result in an unfrozen centre zone. 
The sketch in Figure 4.23a indicates that 
the ambient temperature has been below 
zero for a considerable period resulting 
in the formation of interior ice in the 
tank and frozen concrete throughout the 
thickness of the wall. This is followed 
by a warm period which partially thaws 
the outer part of the wall. The cycle is 
completed by the ambient temperature 
returning to below freezing (in this case 
-10°C) resulting in a thawed centre 
region, an inward moving freezing front, 
and an impermeable interior wall section. 

The increasing hydraulic pressure of the 
pore water trapped in the decreasing 
space between two frozen layers, can 
cause horizontal tensile forces in a tank 
wall. The location of the unfrozen 
water layer will determine the plane of 
failure and dclamination splitting which 



will be manifested as external or 
internal scaling or dclamination. 

Based on this mechanism, the type of 
visible damage occurring depends on the 
extent of the thawing period compared 
to the freezing period, since that 
determines the location of the unfrozen 
central section. Figure 4.24 illustrates 
the nature of the damage which could 
occur depending on the position and 
geometry of the unfrozen section at the 
onset of the freezing cycle. 

4.10 Conclusions and Recommendations 

Based on the review of historical and 
current practices, field observations, and 
the concepts and mathematical models 
described, the following conclusions were 
presented to the task force and the 
Ministry. 



• The detailed surveys confirmed the 
initial work of W.M. Slater & Associates 
Inc. that the deterioration of the 
Province's concrete water tanks was 
widespread. 

• Tank inspections showed that where 
internal coatings existed they could not 
provide the required reduction in 
permeability to prevent saturation and 
consequent deterioration due to freezing 
and thawing cycles. 

• Due to their high internal hydraulic 
pressure, standpipe structures, and in 
particular reinforced concrete standpipes, 
are prone to deterioration by 
dclamination. 

• Small defects which are perhaps 
harmless in other types of structures, 
allow ingress of water into tank walls 
and can initiate dclamination even in 
elevated and ground tanks subjected to 
freezing conditions. 



4 - 22 



From the abo\c conclusions it was 
recommended that: 

1. Methods of providing insulation to 
inhibit the effect of frceze-thaw cycles 
should be studied. 

2. Waterproofing barrier systems for 
potable water storage tanks should be 
studied. 

3. Condition surveys and quality control 
procedures should be initiated as they 
are of the utmost importance during 
tank rehabilitation. They provide 
detailed information on the repairs 
required and ensure that necessary 
quality of materials and workmanship is 
carried out. 



These recommendations were accepted 
and acted upon by the Ministry. Item 1 
and 2 form the basis of other reports 
referenced on page (i). Condition 
surveys, repair methods and quality 
control are discussed in the next 
sections. 



5.0 REPAIR AND REHABILITATION OF CONCRETE WATER TANKS IN ONTARIO 



5.1 General 

Repair is defined as the restoration of 
defects in a structure to the intended 
original state. Rehabilitation is defined 
as repair and upgrading to a new desired 
state, such as designing for increased 
seismic forces, and insulation and 
waterproofing requirements. The tank 
repair and rehabilitation programme was 
more complex and difficult than the 
investigation and diagnostic stage, 
because of the following factors:- 

• Exposure to height (safety) 

• Harsh environment 



failure. The main causes of failure were 
an appreciation of the limitations of the 
materials used, the need for careful 
surface preparation and the provision of 
proper environmental conditions. 
Leakproofing cold joints and deteriorated 
concrete at cracks formed during slip 
forming is difficult and costly; using 
bonded coatings alone is not always 
adequate without removal and 
replacement of the deteriorated concrete 
before surfacing and coating. Complete 
reconstruction of the bottom of walls 
has been necessary at locations where 

water has flowed past or over 
water-stops and where it has frozen on 

the inside. 



• Limited construction season 

• Limitation of suitable contractors, 
specialists, materials 

• Need for temporary storage and 
pumping to ensure continued 
operation of a municipality's water 
supply 

• Limitations of condition survey 
techniques 

• Project management of short term 
contracts 

• Short lead time for the development 
of methods of repair 

• Need for development of efficient and 
safe work stages 

Many of the tanks inspected in the 
period 1981 to 1982 had already been 
unsuccessfully repaired, some of them 
several times before the rehabilitation 
programme started. Reasons for the 
ineffective repairs arose from the lack 
of understanding of the real causes of 



5.2 Design of Repairs 

5.2.1 General 

Remedial measures can be put into four 
categories as follows:- 

• Upgrading of structural integrity 
where necessary; 

• Repair of deteriorated concrete; 

• Containment of water (leakage); 

• Prevention of saturation and freezing. 

It should be noted that the last item 
regarding waterproof coatings and ice 
protection is not fully discussed in this 

report since it forms the basis of other 
reports. 

5.2.2 Structural Evaluation 

.1 Loading 

Loads may be placed in two categories, 
namely applied loads and environmental 



loads as follows: 



Applied Loads 


Environmental 
Loads 


• water pressure (a) 

• post-tensioning (a) 

• wind (b) 

• seismic (b) 


• thermal (b) 

• shrinkage (b) 

• creep (b) 

• ice (c) 



The loads may be described as a) 
well-defined b) defined, or c) ill-defined. 
All these factors, if not handled with 
care, result in stress accumulations 
causing cracking of concrete, 
deterioration of waterproofing and 
severe leakage rendering the tank 
unserviceable in a relatively short time. 

Water pressure is directly related to the 
head of water in the tank, while the 
specified prestress is applied at the 
construction stage. Thus both these 
loadings are well- defined. Wind and 
seismic loads at specified locations in 
the Province of Ontario are defined in 
relevant codes (Ref. 26) and thus the 
levels of such loads for design purposes 
are defined. 

Present design approaches seem to treat 
environmental loading on water tanks in 
a somewhat cursory manner. Some 
aspects are either underestimated or are 
not considered at all. A substantial 
amount of work carried out in New 
Zealand (Ref. 27) on thermal effects 
shows that these are generally 
underestimated due to the neglect of 
direct solar radiation. Thermal gradients 
up to 30°C have been recorded. 
Shrinkage effects are analogous to 
thermal effects and can be treated using 
an equivalent temperature gradient (ETG) 
approach. The ETG is a function of the 
wall thickness and the boundary 
conditions of the walls. Creep is 
generally beneficial in that it tends to 
reduce displacement induced stress. 
However, it also reduces the effective 
prestress in a prestressed tank. While 
such loadings are defined in codes, the 



levels to be considered in design may be 
underestimated. 

Ice loading in tanks is not considered in 
North American approaches but is 
considered in some other countries (Refs. 
28, and 29). Most past work on ice 
pressures has been related to ice sheets 
on lakes and reservoirs and little 
information on its effects in water tanks 
is available. Evidence of ice formations 
in tanks in Ontario exists (Ref. 2). 
These ice formations may be in the form 
of a plate near the top of the tank, a 
cylinder on the inside walls, a slab built 
up at the domed floor of a tank or in 
the cracks in the wall of the tank. 




Photo 5-1 Internal wall ice at bottom of 
tank after demolition in late spring 

Loading from ice formations is most 
critical when the ice heats up, since the 
coefficient of thermal expansion of ice 
is approximately five times that of 
concrete. Work in Finland (Ref. 28) 
shows, that the rate of heating is a 



5 - 2 



significant parameter, and that pressures 
of up to 250 kPa (35 psi) can be 
generated in a tank by a plate of ice 
which warms from -20°C to 0°C in four 
hours. Cracking of an ice plate due to 
differential temperature and subsequent 
freezing forming ice in the resultant 
cracks can develop significant pressures 
700 kPa (100 psi) as shown by work in 
Sweden (Ref. 29). 

Environmental loading on elevated water 
tanks has been studied at Queen's 
University with particular emphasis on 
the effects of ice. This work has been 
reported by T.I. Campbell and W.L. Kong 
in "Ice Loading in Elevated Water Tanks" 
dated April 1986. 

.2 Analysis 

Methods of analysis may be classified as 
(a) simple, (b) adequate, and (c) refined. 
In a simple analysis a tank may be 
modelled as a thin-walled cylinder, while 
in an adequate analysis use can be made 
of charts (Refs. 12, 30, 31 and 32) for 
the computation of the stress resultants. 
A refined analysis can be made using 
classical approaches (Ref. 33) or a finite 
element model (Ref. 34). All methods 
have their use but should be used with 
caution. 

Reinforced concrete tanks have usually 
been designed by taking care of the 
straight hoop forces with the horizontal 
ring reinforcing. This arrangement, for 
water load only, would be suitable if the 
wall at the bottom of the tank were as 
free to expand, as it is for most of the 
tank's height. 

However, in many cases the tank wall is 
rigidly connected to a heavy slab or 
foundation mat. This restraining effect 
completely changes the stress pattern at 
this location. The hoop forces are 
reduced to near zero and their role is 
replaced by significant vertical bending 
moments and horizontal shear forces. 
These stresses rapidly taper off further 



up the wall and in most cases, for all 
practical purposes, can be neglected 
except in the vicinity of the wall slab 
junction where their influence is 
significant. The problem can further be 
complicated by inadequate vertical 
reinforcing. 

In uninsulated water tanks in Ontario 
with large height to diameter ratios and 
cold surface water sources, ice formation 
inside the tanks is a common occurrence. 
The influence of ice pressure, especially 
at rigidly restrained structural modes 
must be investigated. 

Additional problems can be expected due 
to temperature differences through the 
concrete wall thickness. These differ- 
ences can be rather large, considering 
the cold water inside in combination 
with warm outside air together with 
solar radiation. The significant bending 
moments in both the vertical and 
horizontal direction have to be added to 
the other effects already discussed. 

Fast filling of a new tank with cold 
water during a hot summer may cause 
thermal shock and cracking. 

Post-tensioning is applied on the empty 
tank, causing larger than water load 
stresses in reverse. In places of 
restrained freedom of movement these 
stresses can crack the concrete 
before the tank is filled up with water. 
Design analysis will very seldom deal 
with this potential problem. 

In some elevated tanks the post- 
tensioning has been applied in stages, 
while the structural system of the tank 
was changing during the course of the 
construction. For example, the 
cylindrical wall of the tank and the 
tension ring is constructed first and 
partially post-tensioned, followed by 
building of the domed bottom and roof 
and the completion of the remaining 
post-tensioning. 



5 - 3 



It has been noted in cases investigated, 
that considerable temporary bending 
moments are created in locations without 
sufficient reinforcing to take care of 
them. The cracks later appearing at 
such places might have developed in 
these early stages. 

A finite element model shows that a 
standpipe behaves like a thin-walled 
cylinder except in the region of the 
wall-floor intersection where steep 
moment gradients occur in the wall. 
Thus, refined methods of analysis need 
only be carried out in this region 
provided relevant boundary conditions 
are incorporated into such a model. 
Generally, charts are not readily 
applicable to standpipes since most 
charts cover only tanks with floors at 
grade level and having aspect ratios 
outside those of a standpipe. However, 
they may be used provided the designer 
has a proper understanding of the 
overall behaviour of this type of tank. 
The classical approach (Ref. 33) is 
applicable only to tanks with radial 
symmetry. Thermal loading which is not 
symmetrical due to effects of solar 
radiation, wind etc, can only be analyzed 
by a refined model. However, the cost 
of a refined model can be high and such 
models should be used with 
discrimination. 

5.3 Repair Methods Developed 

Repair methods were developed, and 
suitable materials evaluated, in the 
following main areas: 

Structural 

• Vertical crack control 

• Replacement of corroded 
prestressing steel 

• Concrete spalls 

• Delaminated prcstressed wall 
removal 



Leakproofing 

• Steel Liners 

• Epoxy injection 

• Caulking 
Waterproofing 

• Internal coatings (non-toxic, 
odourless, tasteless) 

• External corrosion protection for 
prestressing wires 

• Surfacing materials and 
procedures 



External stage post-tensioning hardware 
and methods have been used for 
replacement of corroded prestressing 
wires in the wall and may be used for 
vertical crack control. 

Ice prevention systems such as 
insulation, circulation mixing, and 
heating are now being used in 
conjunction with structural repairs in 
order to correct cracking caused by the 
expansion force from internal ice 
formations, or where these formations 
are excessive inside the tanks. 



5.4 Tank Repair Methods 



5.4.1 Condition Surveys 

Prior to designing a repair or 
rehabilitation programme it is essential 

that a condition survey is undertaken to 
determine the nature and extent of 

deterioration. The main components of 
the survey are visual inspection and 
exploratory inspection. Record 

photographs should be obtained at all 
stages of the survey. The methods used 

are mapping, light jack hammering, 
coring and laboratory analysis. 



5 - 4 



The first aspect of visual inspection is 
to determine the location of external 
leaks, spalls and cracks. It has been 
found that this is best accomplished 
when the tank is filled. These defects 
should be noted on a plan indicating the 
cardinal points and if possible a point of 
reference such as the manhole. 

The next stages are accomplished after 
emptying the tank and consist of 
external and internal delamination 
mapping and are accomplished using 
hammer sounding. 

It is important that potentially defective 
areas are thoroughly inspected in detail 
to examine the depth of degradation. 
Some of the tank repairs completed 
required further remedial works within 
the maintenance period, which sometimes 
could be attributed to lack of detailed 
inspection at these particular locations. 

Based on current experience, the 
presence of the following possible 
defects should be verified in addition to 
general defects observed: 

• All internal coatings should be 
examined for depth of fracturing on the 
inside concrete surface - obtain cores 
and examine by microscopy. 

• Reinforced concrete standpipes should 
be examined for depth of external and 
internal delamination - cores obtained 
from outside and inside the tank. 

• Gunited tanks should be examined for 
presence of loosely compacted concrete 
and delamination between layers (from 
the shotcrete process). Take cores in 
suspect areas. 

• Post-tensioned, unbonded tanks should 
be examined for the presence of water 
leakage into the tendon ducts - look for 
cracks, water leakage and possible 
corrosion. 



• All tanks should be examined for the 
presence of vertical and horizontal 
cracks indicating possible structural 
weakness caused by ice formation or 
thermal movement or insufficient 
prestress. 

• All construction joints should be 
examined for water infiltration and 
possible deterioration through the 
section. 

• Obtain external and internal cores to 
the depth of the waterstop, particularly 
where leakage is suspected. Verify in 
areas not showing leakage. 

• Determine if jack-rods have been 
used in construction and verify if they 
are completely filled with grout. 

• Locate presence of possible voids in 
tank wall created by improper 
construction methods. 

5.4.2 Surface Preparation 

The first step in all coating 
rehabilitation work is to remove all 

unsound concrete using light chipping 
hammers and sand or grit blasting. 
Water jetting was successful in some 
cases in removing the internal coating 
and fractured internal concrete surface. 

If reinforcing steel is present then it 
can be cleaned using mechanical wire 
brushing and sandblasting. 

5.4.3 Delaminations and Spalls 

Where the concrete is removed to a 
depth greater than 75 mm, welded wire 
fabric reinforcing is included to provide 
mechanical bond. This is accomplished 
by drilling eye inserts and installing ring 
fasteners. The formwork is then placed 
over the patched area and grout poured 
in an entry port. Since only small 
quantities of materials are required, 
proprietary brands of pre-mixed grout 
containing bonding agents are normally 
used. 



5 - 5 



At locations where the depth of removal 
is less than 75 mm, a latex bonder is 
first applied and a stiff mortar is 
trowelled in. 

Curing of all of the above repairs is 
achieved using wet burlap over the 
patched area. 



5.4.4 Crack Repair 

Prior to the determination of the method 
of crack repair, an assessment of the 
overall pattern of cracking has to be 
made. In some tanks vertical and 
horizontal cracks were somewhat isolated 
and cores indicated sound concrete 
within the vicinity of the crack. In 
such cases the procedure used was to 
rout out the crack and fill it with a 
sand filled epoxy mortar. In some 
tanks, parts of the inside surface are 
occasionally highly fissured. Dealing 
with each crack separately was judged 
to be uneconomical and therefore the 
approach taken was to trowel a surface 
layer of epoxy mortar over the fissured 
area. This is usually done in 
conjunction with the provision of an 
overall surface layer as described in 
detail in the report on coatings 
evaluation. 




Photo 5-2 Surfacing tank wall prior to 
application of coating. (HESPELER) 

One difficult area to design repairs for 
is at joints. Typically, joints occur 



between the floor and wall, occasionally 
there are also vertical joints. 
Examination of these joints has shown 
that, in many cases, the water has 
penetrated the waterstop and that 
concrete deterioration is common at 
these locations. 

5.4.5 Waterproofing 

As discussed in previous sections of the 
report, it is important that the water is 
contained within the structure and is 
not allowed to be in close proximity to 
the external surface, as may occur due 
to improper design or defective 
workmanship. 




Photo 5-3 Trowelling latex modified 
mortar. (HESPELER) 

Both bonded coatings and steel liners 
have been used for waterproofing. 



5 - 6 



Where coatings have been applied, it was 
found difficult to provide an intact film 
over the rough surface of the tank wail. 
Consequently, the internal walls of tanks 
undergoing this treatment were surfaced 
with a layer of either modified latex or 
epoxy mortar. 




Photo 5-4 Completed surjocuig prior lo 
coating application. (HESPELER) 



5.5 



Typical Repair Systein.s for \'arious 
Concrete Tank Types 



5.5.1 Concrete Tank Types 

The different categories, designations 
and construction methods used in 
building concrete tanks in Ontario are 
given in Figure 1.1 and Table 1/1. 
These are summarized in Figure 5.1. The 
various construction methods, 



structural types and forms of 
prestressing resulted in a variety of 
defects and forms of deterioration 
requiring individual repair systems to be 
adopted for each tank type. These arc 
described in the following pages with 
respect to each of the tank types 
repaired in the rehabilitation programme 
to the end of 1986. 













a 






E 






a 












m 





3 




caccsc TAiT^S 



Figure 5.1 The 10 Concrete tank types 
in Ontario (see Table 1/1 page 1-3 for 
description and abbreviations) 



5.5.2 RC-S Type Tanks 

Two (2) standpipes of this type, namely 
WATFORD and ALVINSTON were 
repaired, and five (5) have been or will 
be replaced with new steel standpipes 
since the repair cost of the badly 
dclaminated tanks were equal or greater 
than the cost of a new steel standpipc. 



Leaking active vertical (hoop tension) 
cracks at 300 mm(l ft.) approximate 
spacing, apparently static horizontal 
cracks and defective construction joints 
at 600 mm (2 ft.) lifts, had contributed 
to rapid saturation with resultant wall 
delamination and massive external 
spalling in these tanks. The remedial 
methods were to remove and repair the 
delaminated concrete, in one case. 




Photo 5-5 Installation of post-tensioning 
anchors. Note leaks at construction 
joints. (ALVINSTON) 

by casting on a new 200 mm (8 in.) 
thick exterior reinforced concrete wall 
bonded to the old, and closing or 
controlling the vertical cracks from 
opening by external post-tensioning. 
Internal waterproofing was carried out 
using a bonded epoxy coating. 



One standpipe (ALVINSTON) was 
supplied from a surface water source. 
The temperature of the inlet water was 
found to be marginally above freezing 
during winter, resulting in considerable 
ice formation in late winter and possible 
damage due to ice thrust forces. 
Consequently, as part of the remedial 
works the tank was the first in Ontario 
to be insulated and clad with 
pre-finished steel. Additionally, as part 
of the applied research programme the 
performance of the ALVINSTON tank 
was monitored with temperature 
transducers hooked up to a data logger 
and computer. 




Photo 5-6 Installation of external post- 
tensioning tendons. (ALVINSTON) 



5 - 



5.5.3 G-S Type Tanks 



Eight (8) standpipcs of this type have 
been repaired or replaced up to the end 
of 1986, namely BADEN, CHESLEY, 
FENELON FALLS, GRAVENHURST, 
HESPELER, L'ORIGNAL, WINGHAM and 
WOODVILLE. 




Photo 5-7 Typical horizontal cracking 
a G-S type tank. (L'ORIGNAL) 



of 



These types of tanks usually exhibited 
external leakage with incipient and 
actual wire corrosion at locations where 
the cover coat had spalled. The 
majority of these defects were attributed 
to the presence of internal jack-rod 
spalls. Additionally, wall damage and 
horizontal micro- cracking caused by slip 
forming and poor concrete placement 
was present at some of the areas where 
the cover coat had spalled. 

Repairs were effected by removing 
deteriorated internal coatings and 




Photo 5-8 Ring support structure for 
insulation and cladding. (BADEN) 

locating and grouting up the jack-rods 
in the wall. In later repairs these rods 
were more precisely located using 
radiographic methods. Significant extra 
costs were incurred due to the variety 
and extent of wall preparation required 
after removal of old (mainly cementitious 
based) coatings. Bonded epoxy coatings 
applied in 2 or 3 coats to a total 
thickness of 15 to 35 mils were used as 
the waterproof coating on 6 standpipes. 
In some cases a surfacing mortar was 
applied before the epoxy waterproofing 
to fill bugholes and provide a smooth 
surface for the coatings. 

Initial problems were encountered in 
applying effective waterproof epoxy 
coatings in the first rehabilitation 
contracts. These problems were 



condensation of moisture on the walls, 
pinholes in the coating, and failure to 
remove or seal and leakproof weak, 
porous, and micro-cracked substrates. 
Condensation was prevented by 
introducing environmental controls and 
dry heat. Pinholes were eliminated by 
using near 100 per cent solids epoxies 
(solvent free) and using either acrylic 
modified cement or epoxy mortars as a 
parging or surfacing material to fill all 
surface holes, and provide a smooth 
surface layer for the coating. A 
positive air pressure was applied in some 
tanks. 

4 , *•; ^^ 



1^ 




Photo 5-9 Standpipe prior to repair 
(BADEN) 

Concrete deterioration of the walls in 
this type of tank is sometimes difficult 
to identify due to the presence of the 
"confinement" of the prcstressing 
compressive force. This compressive 
force limits typical external delamination 
found in reinforced concrete standpipcs 
but can result in a highly microfractured 
concrete section. For example, in one 
of the repaired tanks, a 5 m high band 




Photo 5-10 Standpipe after insulation 
(BADEN) 




;;^-' ^OOOVl^ 




Photo 5-11 Standpipe after insulation 
(WOODVILLE) 



10 



^SPELER 




Photo 5-12 Stand pipe prior to repair 
(HESPELER) 

of microfractured concrete was not 
identified as a particular problem during 
surface preparation. The sub-strata, 
after scabbling and sand blasting 
appeared to be of suitable quality for 
epoxy coating and this was completed. 
Later, in service, small pin-hole leaks 
formed in the coating and allowed water 
into wall micro-fractures. Several 
months later leachate stains were 
discovered on the outside of the tank. 
The source of the problem was identified 
only after extensive laboratory analysis 
of core samples, and remedial solutions 
were developed. 

It is probable that the initial cracks, 
having a vertical spacing of between 50 
and 250 mm were caused by problems 
with the slip forming process and that 
subsequent cyclic freezing produced the 
fractures within the matrix. 




Photo 5-13 Staudpipe after insulation 
(HESPELER) 




Photo 5-14 Applying coat of MMA to 
exterior of stand pipe (BADEN) 



11 



5.5.4 PTU-S Type Tanks 

GLENCOE standpipe, which is a twin to 
one which failed at DUNNVILLE, has 
been repaired by adding external post- 
tensioning, grouting all the jack-rods 
left in the wall after slipforming was 
completed (located using radiography) 
and internally coating with a bonded 
epoxy. It was later insulated and clad. 
Two (2) failed standpipes were replaced 
with new steel standpipes. 




Photo 5-15 External post tensioning 
(GLENCOE) 



5.5.5 



PTB-S Type Tanks 



One large standpipe of this type in 
Northern Ontario, EAR FALLS, leaked at 
the construction joints at the top of 
each jump-formed lift. The 
rehabilitation method adopted was to 
install a steel liner, grout between the 
liner and existing concrete wall to 



provide a leakproof system, install 
exterior insulation and cladding and 
provide a mixing and heat boosting 
system. 

5.5.6 RC-E Type Tanks 

One tank (BRECHIN) was repaired by 
taking the roof off and installing a 
fabricated internal steel liner. The gap 
between the concrete wall and the liner 
was grouted with portland cement grout 
to provide corrosion protection to the 
back face of the steel. The inside face 
of the steel was coated with vinyl paint. 
External post-tensioning was added to 
control vertical cracking and insulation 
and cladding, for freeze protection. 
Another tank of this type was 
demolished and replaced with a steel 
standpipe (PITTSBURGH). 




Photo 5-16 i'/ct/ liner being installed. 
Note external post-tensioning (BRECHIN) 



12 




Bi- 



Photo 5-17 Tank after rehabilitation 
including strengthening by post- 
tensioning. installation of new steel 
liner, and insulation and cladding. 
(BRECHIN) 

5.5.7 G-E Type Tanks 

The only two (2) elevated tanks of this 
type built with gunite (shotcrctc) walls, 
namely, AMHERSTBURG, and 
CHELMSFORD, have required extensive 
repairs. 




ÏI 




Photo 5-18 Internal maintenance 
inspection of liner paint system after 
one year in service (BRECHIN) 



Photo 5-19 Vertical cracking at base of 
wall due to ineffective prestressing. 
This was caused by inadequate provision 
for inward movement between internal 
thrust ring and wall.(CHELMSFORD) 



Emergency strengthening repairs were 
required to keep them in service because 
of concrete delamination, prestressing 
wire corrosion and breakage, resulting in 
concern for public safety. The leakage 
leading to the wire corrosion in one 
case was due to the presence of 
shotcrete rebound and improper water- 
stop installation resulting in serious wall 
delamination 40 mm (1 1/2 inch) deep. 

Both tanks required major wall repairs - 
in one case an 2.5 m x 1.8 m (8 ft. x 6 
ft.) section was cut out of the 
prestressed wall. The walls of both 
tanks were strengthened with external 
post-tensioning and their horizontal 
floor thrust blocks and rings were 
structurally upgraded to resist earth- 
quake forces. 

Internal waterproofing was carried out 
in both tanks using Tapecrete latex 
modified cement slurry with fabric 
reinforcement. Additionally, a 
mechanically anchored partial liner of 
PVC coated nylon fabric was installed in 
one tank, together with back-up drains 
to substitute for the defective waterstop. 
Some wall leaks re-appeared in one of 
the tanks. 



5 - 13 




Photo 5-20 External post-lensioning 
added to compensate for lack of 
prestressing in wall. (AMHERSTBURG) 

At one of the tanks many vertical voids 
which occurred between the original 6 
mm (1/4 inch) thick cover coat, wires, 
and reinforcing steel were filled. 
Subsequently a 20 mm (3/4 inch) thick 
latex gunite was applied over the wires 
as corrosion protection. In the other 
tank a white MMA exterior coating was 
applied over the thin cover coat to give 
the wires added corrosion protection. 

New sliding aluminum hatches with a 
folding access ladder, to allow lifting 
above winter ice, were added in one 
tank. In the other tank, a plastic sky 
dome was installed replacing an area of 
poor concrete and acted both as a repair 
and to allow more light into the tank. 

Both tanks were insulated and clad in 
1986 and 1987, respectively. 




Photo 5-21 Deteriorated internal thrust 
ring. Note thrust block against wall 
stopping movement. (CHELMSFORD) 




Photo 5-22 Repair of thrust rin^ 
(CHELMSFORD) 




Photo 5-23 Completed repair. Note 
external tensioning at base of tank. 
(CHELMSFORD) 



5 - 14 




Photo 5-24 Tank after leakproofing and 
before insitlationf BRIGDEN ) 

5.5.8 PTU-E Type Tanks 

One (1) elevated tank of this type, with 
hoop and vertical unbonded tendons, has 
been repaired. The BRIGDEN tank 
exhibited leakage at the floor, the 
wall/floor joint, and some wall dampness, 
which was repaired in two stages. An 
interior flexible joint sealant was 
installed by cutting a groove in the 
floor and bonding the sealant to the 
wall. The entire circumferential band 
surrounding the floor joint was given 
two coats of epoxy to waterproof the 
joint. 

The first stage was completed prior to 
the winter of 1983, and included 
temporary repairs to the damp buttress 
recesses holding the unbonded tendons 
and to leaks in the tank roof. 




Photo 5-25 Steel support system for 
insulation and cladding. (BRIGDEN) 

During the second stage, completed the 
following spring, poor quality concrete 
mortar was removed from all the post- 
tensioning anchor recess pockets, which 
revealed a number of slack anchorages 
due to failure of five of the unbonded 
prestressing tendons. Detailed 
investigation demonstrated that the 
strands had broken as a result of stress 
corrosion. The corrosion was caused by 
water entering the anchorages through 
the porous mortar and into plastic tubes 
sheathing the strands which were 
inadequately protected by corrosion 
inhibiting grease. The inhibitors in the 
grease may have been rendered inactive 
or the grease itself may have been 
displaced by the infiltrating water. (See 
Section 6 for more details). The failed 
strands were removed, replaced with new 
regreased strands, and the recess 
pockets were filled with a dense 
non-shrink reinforced mortar. 



5 - 15 




Photo 5-26 Completed rehabilitation of 
tank. (BRIGDEN) 

During the leakage test for the stage 2 
repairs, dampness was again observed at 
the wall/floor joint which had been 
sealed during the previous stage 1 
repair. This included removal and 
replacement of areas of deteriorated 
surface mortar. 

Investigation revealed that although the 
surface mortar was "sound", deteriorated 
concrete existed underneath and close to 
the waterstop situated at the centre of 
the wall, and was hidden by the mortar. 
All unsound material was removed, 
except adjacent to the vertical post- 
tensioning dead end anchorages, and 
further repairs were carried out to 
correct the original fault of a poorly 
installed waterstop. The bottom of the 
wall was re-constructed using epoxy pea 
gravel and sand mortar. 

It was considered that the inferior 
material at the centre of the wall was 
the end product of an improperly 
installed waterstop which allowed the 



mortar to saturate and whose structure 
had subsequently been destroyed by 
freezing and thawing. 

This tank was insulated and clad as the 
final stage of rehabilitation. 

One other tank of this type was 
demolished and replaced by a new steel 
tank (VERNER). 




Photo 5-27 Delaminated exterior at 
source of leak after removal of fractured 
concrete. (CASSELMAN) 

5.5.9 PTB-E Type Tanks 

Eight (8) tanks of this type were 
constructed. The 1981 study indicated 
that these tank types had one of the 
highest performance ratings, exhibiting 
the least amount of visible deterioration. 
Consequently, their repair was scheduled 
for the latter stages of the programme. 
After the 1981 external inspection, three 
(3) tanks developed serious deterioration 
problems and were repaired earlier than 
anticipated. 

In 1984/85 winter the CASSELMAN tank 
suddenly developed a small wall leak 
adjacent to an ungrouted post-tensioning 
duct. Detailed investigation revealed 
that the wall was delaminated at that 
region of the tank. The remedial 
solution adopted for this tank was to 
demolish the walls and roof of the tank, 
and to construct a new steel tank on 
the existing base. 



16 




...MP-.. mh ' \l 
Photo 5-28 Dclaminaled wall. Inspection 
revealed ungroiited post-tensioning duels 
at this region. (CASSELMAN) 



The PICKLE LAKE tank was noted to be 
badly cracked and delaminated by 
internal ice forces after the 1982/83 
winter. The remedial solution designed 
for this tank was to construct a steel 
liner inside the existing concrete tank 
and to insulate the space between the 
steel and concrete walls. 

Although RED LAKE exhibited little 
external deterioration, the 1985 internal 
inspection revealed considerable 
delamination of the inside walls of the 
tank. A remedial solution similar to the 
PICKLE LAKE repair system was selected 
for this tank. 

The remaining five (5) tanks of this type 
have exhibited only minor deterioration. 
The rehabilitation solutions adopted for 
these tanks have included the application 
of cementitious or epoxy coating to the 
inside concrete wall, and the installation 
of external insulation and cladding. 

5.5.10 G-G Type Tanks 

Two (2) wire wound gunite ground tanks, 
namely BARRY'S BAY and ORILLIA, 
were repaired because of excessive 
leakage - one through the floor, the 
other through the walls. One of the 







Aiiîy-i- ■* 'i?^ "^ 




m.,.. 


x 











Photo 5-29 Reservoir before repair 
(PRESTON) 




Photo 5-30 Reservoir after repair 
(PRESTON) 

tanks which had extensive floor cracking 
was sealed and Tapecrete was applied 
over the floor area and around the 
entire region of the wall/floor joint. 
The other tank, a large municipal tank, 
was waterproofed with the Tapecrete 
fabric and slurry system. 

The smaller of these tanks, BARRY'S 
BAY, exhibits an ice formation in late 
winter and will be insulated and clad. 

5.5.11 RC-G Type Tanks 

CAMPBELLFORD tank was internally 
coated using a cementitious slurry and 
fabric method and was insulated and 
clad. 



5 - 17 




? s;»-' 



Photo 5-31 Tank demolition - culling 
hole at base. Note ice still inside empty 
slandpipe. (CALLANDER) 




Photo 5-32 Standpipe toppled. 
(CALLANDER) 




Photo 5-33 Reinforced concrete tank 
wall after toppling. Note no corrosion of 
steel and delamination of concrete from 
steel. (CALLANDER) 



5.6 Quality Assurance and Measurement 

As stated at the beginning of this 
report, field observations soon indicated 
that the Ontario water tanks were 
located in a very severe environment. 
Small defects in the tanks, perhaps 
insignificant in other types of 
structures, have resulted in rapid 
deterioration of the tank or at least a 
significant part of the tank. It was, 
therefore, considered that quality 
assurance of the remedial works was of 
the utmost importance to assure success 
of the repairs. Resident inspection of 
the remedial work was completed on a 
100 per cent basis. The inspection was 
supplemented by routine testing and 
occasional specialist advice and testing. 

During the repair programme, several 
quality assurance issues arose. These 
related to humidity, concrete wall 
temperature, surface preparation, coating 
thickness and bond strength 
measurement. 

In order to check that sand or 
grit-blasted surfaces were prepared to 
the desired roughness, test patches were 
prepared and compared with a selected 
grade of sandpaper with grit size (ALO 
80). This is reported in the coatings 
report. 

Sites were issued with sling 
psychromcters and rotary bi-metal 
thermometers. It was found that the 
rotary thermometers were inadequate to 
measure air or wall temperatures with 
sufficient accuracy. However, a rapid 
response surface electronic thermometer 
proved successful. 

Coating thickness was initially measured 
using wet thickness combs and gross 
volumetric measurement. Where coatings 
had solvents or were applied on rough 
substrates, the wet thickness gauges 
proved to be inadequate. Additionally, 
on rough surfaces, although measurement 
of the consumption of gross quantity of 



5 - 11 



materials accurately reflected the 
average thickness, microscopic analysis 
of core samples showed that in many 
areas the coating was extremely thin. 
Consequently, a non-metallic coating 
thickness gauge was introduced. This 
test essentially involves scratching the 
coating down to the concrete interface 
with a cutter of known grooving angle. 
Measurement of the dimensions of the 
groove, and hence coating thickness are 
obtained using an inbuilt measuring 
microscope. It was found that 
measurements of the minimum coating 
thickness (the specified measurement) 
could be determined rapidly by the site 
inspector. 

Measurement of bond strength cither of 
the cementitious based repair materials 
or the coating itself was an important 
consideration, and therefore, during the 
course of the 1983 programme, test 
methods were developed which could be 
used to assist the repair programme. A 
modification to the Lok-test apparatus 
was constructed. The Lok-test is 
essentially a hydraulic ram which exerts 
a pull normal to the test surface. To 
test the repair materials, a 50 mm 
(2 in.) diameter diamond core drill was 
used to make a cut extending beyond 
the repair material. A steel disc was 
attached to the surface using rapid 
setting epoxy resin and was pulled in 
direct tension using the modified 
Lok-test apparatus. 

To test the bond strength of the coating 
to the sub-strata, a similar technique 
was used. However, for this test, the 
initial core cut was not required. 



Time domain reflectometry was used to 
monitor the grout and water levels. 
This technique uses guided electrical 
pulses and is sensitive to the medium 
surrounding the wire. Wires were 
positioned to the height of the liner at 
cardinal points and connected to an 
oscilloscope at ground level. Using a 
series of switches it was possible to 
monitor the grout and water level by 
observation of the oscilloscope at ground 
level. This system also enabled control 
of pumping rate and sampling for 
percent bleeding and compressive 
strength to be accomplished at one 
location. 

The above practical techniques were 
found useful to avoid some problems 
encountered in the tank repair 
programme. High humidity and low wall 
temperatures have initiated the forma- 
tion of condensation, with resultant 
blistering of the coating, after one year 
of service. Cold wall temperatures have 
made the application of epoxy resin 
coatings difficult due to a substantial 
increase in viscosity at low 
temperatures. Rough surfaces have 
produced coatings with an uneven profile 
with the consequent necessity to apply 
additional coats. The systematic 
application of some of the above 
techniques, therefore, cannot be 
overemphasized and will result in 
benefits to both the contractor, and the 
consultant in addition to assuring an 
acceptable repair. 



Additional quality assurance procedures 
were required during the grouting of 
steel liners. Due to the use of thin 
liners it was necessary to avoid high 
fluid pressures occurring in the grout 
which could buckle the liner. One 
procedure used, was to balance the fluid 
pressure by filling the tank with water 
at a similar rate to the rate of grouting. 



5 - 19 



6.0 



DETERIORATION OF METALS IN CONCRETE WATER TANKS 



6.1 



Introduction 



6.1.1 Role of Metals In Concrete Tanks 

The main part of the report is focussed 
on the deterioration of concrete as a 
material, and on the reasons for the 
need to rehabilitate defective reinforced 
and prestressed concrete water tank 
structures. Metals, however, especially 
steel, provide important and necessary 
structural components of the concrete 
tanks. These metals, as explained in 
this section, can corrode, and in some 
cases, critically weaken the structure. 
In addition to the steel reinforcing bars 
and post-tensioning tendons cast in the 
concrete wall itself, prestressing wires 
or strands are also wound around the 
walls and protected with gunite 
shotcrete. This steel reinforcement 
provides the principal structural tensile 
reinforcement for concrete tanks; 
however, many other important concrete 
tank components, vital to operations, are 
fabricated from metals, mainly steel and 
aluminum. 

Some of these other metal components 
are access manways and covers, roof 
beams, decks, hatches and covers, 
elevated tank floor beams, air vents, 
internal and external ladders and 
landings, safety rails, inlet and outlet 
pipes and valves, floor drains and 
covers, overflow pipes and supports, air- 
craft lights, and more recently, elements 
of freeze protection systems such as 
mixing units, temperature sensors, and 
roof hoists. All these tank 
appurtenances are subject to 
deterioration and must be maintained in 
good and safe working condition in 
order that a concrete water tank can be 
operated satisfactorily, inspected, and 
maintained efficiently and safely. 



6.1.2 Deterioration Sequence 

An important point which has resulted 
from the inspection of many tanks 
during the Ministry rehabilitation 
programme is that the inspections of 
leakage and concrete deterioration 
have generally, incidentally, led to the 
discovery of the corrosion and 
deterioration of metal components in 
concrete tanks. Corrosion has been 
observed in critical components in tanks 
in service for less than 8 years. Since 
these components are often vital for the 
safe operation of the tank, it is 
important that the problem is rectified 
as soon as possible. In a planned 
maintenance programme, the condition of 
all metal components should be inspected 
in a separate scheduled programme. 

6.1.3 Importance of Construction 
Process 

Another important factor which must not 
be ignored, is the process by which 
post-tensioning is installed in the 
concrete walls. The steel tendons are 
placed in steel or plastic tubes so they 
can stretch during tensioning. Water 
entering through defects in these hollow 
tubes can freeze and expand, cracking 
the concrete section. In some cases, 
this can lead to sudden leakage problems 
and even tank failure. This is neither a 
direct concrete material, nor a metal 
deterioration problem, but a problem 
resulting from the construction process 
itself. 

6.1.4 Summary 

On a cost comparison basis, corrosion of 
steel and deterioration of other metals 
has not been as serious a problem in 
concrete tanks in Ontario to date, as 
has been the problem of concrete 
deterioration in the freezing 
environment. Recent observations, 



6 - 1 



however, indicate that the corrosion 
problem is increasing and cannot be 
ignored in a rehabilitation and mainten- 
ance programme. 



TABLE 6/1 
Types of steel reinforcement 



6.2 



Corrosion of Steel Wall 
Reinforcement 



6.2.1 Need for Reinforcement 

Steel reinforcement provides the tensile 
strength required at ultimate and service 
loads to resist the applied loads from 
the retained water, wind and earthquake, 
as well as environmental loads, in all 
concrete water tanks. Concrete, being a 
brittle material, cracks at a very low 
tensile strain. Sufficient steel 
reinforcement, therefore, must cross an 
incipient or actual crack to resist the 
load and control the crack width. 
Alternatively, sufficient prestress must 
be supplied by post-tensioning tendons 
to pre-compress the concrete to balance 
and resist the tensile load, and for 
leaktight construction, prevent the 
concrete from cracking. Concrete and 
steel materials must, therefore, be 
combined in the construction process to 
result in a load resisting structure. 

6.2.2 Types of Reinforcement 

Four types of steel reinforcement 
consisting of two grades - reinforcing 
steel and prestressing steel, have been 
used in the construction of the 53 
concrete tanks in Ontario, and are listed 
in Table 6/1. 

All 3 types of post-tensioning use high 
tensile prestressing wires on either an 
individual basis (G), or as strands of 7 
wires (PTB and PTU). All prestressed 
concrete water tanks, including the G 
type, include some ordinary reinforcing 
steel, as well as tendons, in the 
concrete wall. 



No. of 


Tank 


Description 


tanks 


designation 




built 






12 


RC 


Reinforcing 
bars 


12 


PTB 


Post-tensioned 
tendons (bonded) 


9 


PTU 


Post-tensioned 

tendons 

(unbonded) 


20 


G 


Post-tensioned 
wires, wound and 
gunite protected 


53 To 


al 





6.2.3 Detection of Corrosion 

The mechanism of general corrosion of 
steel resulting from the development of 
an electrical battery with an anode, a 
cathode, and an electrolyte, is described 
in many texts and will not be described 
here, except where special circumstances 
pertaining to concrete tanks occur. 
Half-cell measurements using equipment 
consisting of a copper/copper sulphate 
cell can, in some circumstances, detect 
the level of corrosion activity of steel 
within a concrete wall from electrical 
potential readings. "Hot spots" where 
electrical potential readings over 350 
milli-volts are read can lead to locations 
where active steel corrosion within the 
concrete may be occurring. However, 
this method must be used with caution 
to survey the corrosion state of 
prestressing wires. In the walls of 
prestressed tanks, steel reinforcing bars 



6 - 2 



in vertical and horizontal directions, as 
well as vertical steel jack-rod pipes, may 
exist in the wall in addition to the 
prestrcssing wires. Readings of 
corrosion "hot spots" in the wall can 
lead to conclusions that the initial 
prestrcssing wires are corroding, when 
in fact, investigations by chipping into 
the wires and the wall may indicate that 
the wires are bright and corrosion free 
on the outside, but that the corrosion is 
taking place at an unimportant jack-rod 
coupling near the inside face of the 
wall. It is, therefore, important to 
combine half-cell investigations with 
visual examination of corrosion at 
"hot-spots" by removal of concrete, 
before conclusions are made as to what 
type of steel is corroding in the wall, 
and the actual location and state of the 
corrosion. 

In most cases of steel corrosion 
observed in concrete tanks, except in 
the case of unbonded tendons, signs of 
corrosion are usually evident, and can be 
seen as rust, pits or rust stains on the 
concrete surface. Where the concrete is 
delaminating, atmospheric corrosion is 
sometimes observed after removal of the 
concrete. 

6.2.4 Protection of Steel by Quality 
Concrete 

It has been stated previously that 
general corrosion of steel reinforcement 
in concrete has not been a serious 
problem in the concrete tanks inspected 
except at locations where there is 
leakage and dampness, exposure of the 
steel to the atmosphere, or at a visible 
concrete delamination, spalling or porous 
area of concrete. Good quality concrete 
with a maximum water/cement ratio of 
about 0.50 and an adequate Portland 
cement content, provides a protective 
passive alkali environment for steel 
reinforcement, prestrcssing wires and 
strands. The pH (hydrogen ion level) on 
the O (acid) to 14 (alkali) scale, with 7 
being neutral, should be a minimum of 



12 in concrete in order to provide 
permanent protection of steel against 
corrosion. The pH value of poor quality 
and porous concrete can be reduced by 
carbonation (COj), acid rain, and an 
aggressive environment so that the 
original protective passive property of 
the alkali concrete material is lost, and 
the corrosion of steel can start. 

The widespread corrosion and concrete 
delamination caused by chlorides from 
de-icing salts as experienced in concrete 
bridge decks and parking garages in 
recent years, is noticeably absent in 
concrete water tanks in Ontario. 

6.3 Observation and Repairs 

6.3.1 Reinforced Concrete Tanks 
(Type RC) 

Corrosion of the reinforcement in 
reinforced concrete (RC) tanks which 
are non-prestressed has been negligible. 
Even where external spalling of the 
concrete has occurred causing the steel 
to be exposed for years, the general 
corrosion has not been serious 
(WATFORD, CAMLACHIE, ALVINSTON, 
etc.). 

Examination of reinforcement adjacent to 
vertical delaminations 1 mm wide in the 
walls of concrete tanks under perma- 
nently saturated conditions has revealed 
no sign whatsoever of corrosion of the 
steel with approximately 30 mm of con- 
crete cover (ALVINSTON, CAMLACHIE). 

Examination of the reinforcement from a 
large elevated reinforced concrete tank 
demolished after less than ten years in 
service (PITTSBURGH) revealed that 
there was no corrosion and that the 
steel was in an "as new" condition. 

All observation confirms that a 
protective cover of about 25 mm of good 
concrete provides satisfactory corrosion 
protection for reinforcement during the 



6 - 3 



normal expected service life of 30 to 50 
years of a concrete water tank 
(PRESTON - 30 years old). 

6.3.2 Post-tensioned Bonded Tanks 
(Type PTB) 

.1 Description 

Post-tensioned bonded tendons in 
Ontario concrete water tanks consist of 
a single 16 mm diameter 7 wire 
prestressing strand placed in 30 mm 
diameter corrugated ducts or sheaths 
manufactured from plain bright steel 
strip. 

After tensioning the strands, protection 
against corrosion is carried out by 
injecting a neat portland cement grout, 
containing an expansion admixture, into 
the ducts with a pressure pump, so they 
are filled with alkali material with a 
minimum pH of 12. 

The grout then hardens, thus effectively 
bonding the strands in the corrugated 
ducts within the structure like normal 
hi-bond reinforcement in concrete 
material. 

Hoop or circumferential tendons are 
generally 180°, (half circle) with steel 
anchorages at the ends installed in 
external buttresses or internal pockets. 
The strands are locked off after 
tensioning (and before injection) in 
tapered holes in the steel anchorages, 
with tapered wedges, and with machined 
teeth. Straight vertical tendons are 
incorporated in some PTB tanks. 

.2 Problems (See Figure 6.1) 

Ungrouted ducts 

Problems can occur later if the ducts 
are not properly filled with portland 
cement grout by injection after 
tensioning the strands. As described 
elsewhere, the empty ducts may fill 



with water some years later and freeze 
in winter, causing expansion and 
cracking of the wall (EAR FALLS, 
CASSELMAN). 




Figure 6.1 Bonded tendons. Bleed void, 
corrosion of unprotected prestressing 
steel. Concrete anchorage pocket plug 
shrinks (large circle, upper left) and 
becomes loose. Poor bond or non- 
expansive mortar permit aggressive 
materials access to anchorage and 
prestressing steel, likewise with 
improperly bonded and anchored 
exterior-end anchorage (shown at left 
end of prestressing steel) protection, 
(ref. 35) 

This can lead to increased leakage in 
the spring followed first by corrosion of 
the sheet steel ducts and then corrosion 
of the unprotected and vital prestressing 
strands themselves. The deterioration 
process here, as described elsewhere in 
the report, is similar and occurs in 3 
stages. First the initiation stage 
consists of the steady breakdown of the 
waterproofing system followed by 
concrete saturation and finally leakage 
into the empty ducts. The second active 
stage consists of the filling of the ducts 
with water, freezing, cracking of the 
concrete section and subsequent leakage 
of water to the outside. Exposure to 
air, together with moisture, initiates the 
start of corrosion of the duct and 
prestressing strand. The third and final 
stage is the loss of steel section and 
prestressing force, eventual failure of 



6 - 4 



the strands or wires and loss of strength 
of the tank wall section. 

Investigations under the spalled concrete 
fill in the vertical tendon anchorage 
recesses at the roof level of one 
standpipe (MILLBROOK) revealed severe 
corrosion of the strands and wedge 
anchorages. Some strands had been 
completely eaten away down into the 
corroded wedges to the point where 
anchorage of the strand tendon by grout 
bond was essential, because the steel 
anchorage had failed. The severity of 
the aggressive corrosion into the 
anchorage indicated that calcium 
chloride, or a similar corrosive material, 
may have been used in the mortar to 
either speed up hardening of the mortar 
or prevent it from freezing. 

.3 Repair 

Diagnosis of the strand corrosion 
problem (a) described above, by 
investigating at the leak locations, must 
be made early so that the corrosion 
process can be stopped before 
significant damage is done. This can be 
accomplished by filling the ducts with 
protective epoxy or cement grout before 
the loss of section and the state of 
corrosion of the wires of the strands 
becomes serious from pitting, or if 
stress corrosion occurs. 

If strands are seriously corroded or have 
failed, it may be necessary to strengthen 
the tank with added external post- 
tensioning tendons, as well as 
leakproofing the tank. In this case the 
designer must check that the wall is not 
overstressed in the empty tank state by 
too much additional prestress. 

6.3.3 Post-tensioned Unbonded Tanks 
(Type PTU) 

.1 Description 

Post-tensioned unbonded tendons in 
Ontario concrete water tanks consist of 



single 13 or 16 mm diameter 7 wire 
prestressing strands, coated with 
protective water-resistant grease charged 
with rust inhibitors, and pushed into 
black polyethylene tubes with walls 
about 2.5 mm thick. The air space 
between the coated strand and the inside 
of the tube may be about 2 mm. Steel 
anchorages are installed at the ends of 
each 180° or 360° hoop tendon to lock 
off the strands with tapered wedges with 
machine teeth in tapered holes in the 
anchorages. Straight vertical tendons 
are added in the walls of many PTU 
tanks. If either the strand or an 
anchorage fails, the total unbonded 
tendon fails and is lost to the structure. 




Figure 6.2 Unbonded monostrand. The 
anchorage plug shrinks and becomes 
loose. Poor bond and non-expansive 
mortar permit aggressive materials access 
to anchorage and prestressing steel. 
Strand portion is exposed to concrete 
because no physical connection is made 
between the sheath and anchorage. At 
stressing end. this portion of tendon is 
pulled through intimate concrete closure 
when stressed. Tie wire between 
perpendicular tendons causes local 
indentations in sheaths which tend to 
shear off when tendons are tensioned. 
Reinforcing bar indentation causing hard 
point that tends to shear off when 
tendon is tensioned. 



6 - 5 



.2 Problems (Sec Figure 6.2) 

Two problems exist with unbonded 
tendons, firstly that of corrosion of the 
strands which is the subject of this 
section; and secondly, the filling of the 
air spaces in the plastic tubes with 
water under pressure, and freezing, 
especially of vertical tendons. This may 
contribute to the rupture of the wall 
causing complete and sudden failure of 
tanks (DUNNVILLE and SOUTHAMPTON) 
which is d'^scribed elsewhere in this and 
other reports. 

Five (5) of the 116 unbonded strand 
tendons were found to have failed by 
stress corrosion cracking during the 
rehabilitation of the seven year old 
BRIGDEN elevated concrete water tank. 
For a typical failure see Photo Nos 6-1, 
6-2, 6-3 and 6-4. The sequence of 
events were presumed to be as follows: 




• water from external precipitation 
penetrated to the tendons through the 
poor quality and porous concrete fill in 
the anchorage recess pockets. 

• stress corrosion cracking across 
some wires started at the 
non-metallic inclusions in the steel 
surface. 

• longitudinal brittle fractures in the 
wire started at the stress corrosion 
crack sites as a result of bending 
stresses on the wire due to the wall 
curvature. 

• tensile brittle fractures of the 
remaining uncorroded wires occurred 
similar to that expected from an 
increased load on the remaining wires. 




Photo 6-1 Poorly protected post- 
lensioning anchorage allowing water to 
enter. (BRIGDEN) 



Photo 6-2 Detailed view of strand 
showing corroded wires. (BRIGDEN) 



.3 Repair 

All anchorages were exposed by 
removing the covering mortar from the 
recesses. Lift-off load tests were 
carried out on all but 4 of the 116 
strands in the tank with specially 
developed tools and post-tensioning jacks 
to a load of 70 per cent of the 
guaranteed ultimate strength of the 



6 - 6 



strands which was 17 per cent above the 
design load of the strands (60 per cent 
ultimate). This action resulted in 
obtaining a proof test of the total hoop 
strength of the tank at that time and 
identifying that all 5 strands failed from 
corrosion and were no longer capable of 
supplying the proof test force slightly 
above the required design strength of 
each strand. 




Photo 6-3 Typical longitudinal fracture 
of one of the wires of a strand. Lower 
break shows some elongation which was 
not typical of breaks observed. Some 
grease is present on the wire. 
(BRIGDEN) 

Techniques were developed for removing 
the 5 strands, either already failed or 
failing to meet the proof test load, and 
replacing them with new strands coated 
with grease in the existing plastic ducts. 

Finally the anchorages and recesses were 
sand-blasted, mesh reinforcement 
fastened with drilled inserts was 
installed, and a high quality latex 
concrete mortar was used to fill the 
recess pockets to seal the ends of all 
tendons and prevent further water 
leakage into the ducts. 



6.3.4 Gunite Protected Tanks 
(G Type Tanks) 

.1 Description 

The sequence of construction of 
post-tensioned wire wound (G type) 
tanks is different to that of PTB and 
PTU types described previously, where 
the strand post-tensioning tendons inside 
ducts are placed first, then cast into the 
concrete walls. After the tank wall is 
completed and when the concrete 
reaches a specified minimum compression 
strength, normally 24 to 28 MPa, the 
tendons are tensioned and anchored. 
This is carried out in a specified order 
of stressing so that the wall is not 
cracked by the applied initial 
prestressing forces during the 
post-tensioning process. 




Photo 6-4 Start and progression of a 

typical fracture. Fracture starts at the 
surface of the wire then progresses 
longitudinally going deeper into the wire 
as it progresses. Note corrosion of wire 
near the bottom of picture. An example 
of the second type of fracture 
(transverse) can be seen in the wire on 
the right of the strand. (BRIGDEN) 



In the G type tanks, the wire tendons 
are tensioned as they are pulled through 
a die and wound continuously around the 
already constructed concrete tank wall 
after it reaches the required minimum 
strength, usually 31.5 MPa. The walls of 



6 - 7 



ground and elevated G type tanks are 
normally constructed of shotcrete 
concrete (gunite) mortar. All standpipes 
in Ontario, except one, were constructed 
using slipformed concrete. This method 
uses vertical jack-rod pipes, coupled 
every 3 m (10 feet) height in the walls, 
on which the forms are continuously 
raised by hydraulic jacks during the 
concreting process. The jack-rod pipes 
left in the walls have caused serious 
concrete deterioration problems in many 
tanks because of internal spalling at the 
coupling points due to the pipes filling 
with water and then freezing, causing 
explosive forces on the concrete. This 
deterioration is described elsewhere in 
this report. 

The hoop post-tensioning of G type 
concrete tanks is carried out using No. 8 
gauge high tensile prestressing wire of 
4.1 mm diameter drawn through a die to 
3.6 mm. The initial wire stress is 980 
MPa reducing to a final effective stress 
of 735 MPa after all losses. After the 
post-tensioning of each layer of wires is 
completed, they are covered with a thin 
2-3 mm wash layer of gunited on 
concrete mortar (shotcrete). The 
number of layers of prestressing wires 
placed on top of previous layers is a 
function of the hoop force from the 
wires required to resist the applied 
water load, and any other design loads 
on the tank. A maximum of five (5) 
designed layers has been observed. 
Additional layers to the designed number 
have been applied in some tanks 
(L'ORIGNAL and FENELON FALLS) 
because of problems during post-tension- 
ing, or with the quality of the wire. 

After the completion of winding on the 
final exterior layer of wire, the wires 
are protected with a final coat of the 
pneumatically applied concrete gunite 
mortar. This cover coat thickness is 
usually specified as either a minimum of 
20 or 25 mm. 



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V' ■■■■ ■'.'-. 

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Figure 6.3 Flaws leading to corrosion of 
wire wrapped circular pipes and tanks. 

A) sand pockets or voids which permit 
the passage of electrolytes and oxygen. 

B) uncontrolled structural cracks, 
permitting the environment access to the 
prestressing steel. 

C) Bundled wires or strands which can 
result in a continuous void. 

D) Delaminated cover coat which is 
generally connected with a 
circumferential crack exposing the backs 
of the prestressing steel. 

E) Inadequate cover coat which is less 
than 15 mm or pervious, permitting 
access of environmental contaminants. 

F) Cracks or honeycomb in cylinder core 
giving access of contaminated material 
to prestressing steel. 

G) Electrical connection of prestressing 
steel to other metal components. 

Note that electrolyte has to be present 
for corrosion to occur 

.2 Problems (See Figure 6.3) 

Two elevated G type tanks were built in 
Ontario, at AMHERSTBURG, and 
CHELMSFORD. Both these tanks 
developed severe wire corrosion problems 
at the bottom of the tank walls for 



6 - 8 




Photo 6-5 Delaminated cover coat 
exposing broken prestressing wires 
(AMHERSTBURG) 

heights of 2 m above the floor slab and 
required major repairs, including 
strengthening by external 
post-tensioning, after about ten years in 
service. 

Seven broken wires were found at the 
AMHERSTBURG tank and a further 
number badly corroded in an area 2 m x 
2 m adjacent to a major leak at a wall 
delamination or split and where the 
cover coat had fallen off, exposing the 
wires, (see Photo No's 6-5, 6-6) The 
original cover coat was only about 5 mm 
thick over the wires. 

A similar area of corroded wires, to that 
described above, was located at 
CHELMSFORD, when a large area of 
de-bonded cover coat which had bulged 
75 mm outward, was removed. The 
de-bonding had exposed the wires to the 
elements probably soon after the tank 
was filled, judging by the severe state 
of the corrosion, and other observations. 
Vertical cracks in the tank indicated 
that strengthening was required. 

A number of broken or severely 
corroded wires have been found in three 
(3) G standpipes, generally at locations 
of local leakage and where the thin 




Photo 6-6 Corroded prestressing wires 
under delaminated cover coat 
(CHELMSFORD) 

cover coat has de-bonded and fallen off 
the vertical face exposing the wires. 

Figure No. 6.3 shows flaws leading to 
corrosion of wires of wire wrapped tanks 
(after Schupack). 

.3 Repair 

Two external post-tensioning procedures 
have been used for strengthening tanks 
with corroded or failed circumferential 
prestressing wires, both using special 
anchorages developed for tanks. 

The first method was to install greased 
strands in black polyethylene tubes 
around the tank. The second method 
was to place bare strands around the 
tank with the stressing anchorages near 
the broken wire. This assures that the 
maximum force is applied near the 
break in wires, because of the 
steel/concrete friction loss around the 
tank. The strand is then protected from 
corrosion by spraying on a cover of 
gunite. 

Care must be exercised in the design, 
specifications, and the application of 
post-tensioning not to overstrcss the 
tank wall so that it cracks in the empty 
state. 



6 - 9 




obtained in 2 coats (with some touch up) 
enhancing the appearance of the repair, 
which is important in the repair of 
concrete tanks because of their high 
profile, (see photos 5-23 and 5-30) 



Photo 6-7 Corroded access lube 
(PRESCOTT) 

Corrosion of prestressing wires in G 
type tanks from exposure to external 
moisture and air through a cover coat 
which is too thin ( much less than the 
recommended 20 mm minimum), presents 
a difficult maintenance problem. If 
corrosion is general, widespread and well 
advanced, or a number of broken wires 
exist, strengthening by adding external 
post-tensioning may be possible. If the 
corrosion is minor and sporadic, extra 
protection to the wires by adding 
surface coatings can be carried out. An 
extra gunite layer, preferably a 20 mm 
additional thickness of Portland cement 
mortar, modified with latex, or a methyl 
methacrylate (MMA) coating, which is 
less costly, have been used. The MMA 
coating has been developed so that an 
attractive solid white gloss finish can be 




Photo 6-8 Corroded roof truss 
(HESPELER) 




Photo 6-9 Completed rooj repair 
(HESPELER) 



10 



6.4 Deterioration Of Metal 

Components 

6.4.1 Steel Access Tubes in 
Elevated Tanks 

.1 Problem 

Three (3) elevated concrete tanks in 
Ontario, AMHERSTBURG, CHELMSFORD 
and BRIGDEN, are constructed with 
internal steel tubes containing access 
ladders to the roof, or into the tank, 
and provide support for the roof. These 
steel tubes are primary tank components 
and must be protected from corrosion so 
that their strength and leaktightness are 
maintained, or the tanks will fail to 
retain water. 




Photo 6-10 Aluminium ladder exhibiting 
severe pitting corrosion (BRIGDEN) 



.2 Repair 

Severe corrosion and pitting of the 
central steel access tube in contact with 
the water occurred in the BRIGDEN tank 
after six years in service. Protection 
was carried out by removing the rust 
and preparing the steel by grit blasting 
and re-painting with a five coat vinyl 
paint system as specified for new 
Ministry steel water tanks. 



6.4.2 Aluminum Ladders in Water 

.1 Problem 

Severe, deep, and widely distributed 
pitting of the internal aluminum ladder 
below the water line was observed in 
the BRIGDEN tank described above, with 
a central corroded steel access tube. 

Investigation of the corrosion showed 
that the pits were initiated on the 
surface of the aluminum because of the 
central presence of iron on the surface. 
Iron was present in the water because 
of the corroding central steel tube. 
Small particles of iron set up localized 
galvanic cells which resulted in pitting 
corrosion over 10 mm deep in places. 

.2 Repair 

As it was virtually impossible to 
completely stop the advanced pitting 
corrosion in the aluminum it was 
recommended that the ladder be 
replaced. It was decided to replace the 
ladder with one fabricated from steel 
and hot-dip galvanized for protection. 

6.4.3 Recommendations 

• There have been reports of certain 
types of aluminum corroding in 
chlorinated water. 



11 



• If aluminum is used, it is 
recommended that it be an aluminum 
alloy such as Alclad 6061-T4 or T6 
temper.. 

• Some authorities are now specifying 
fiberglass rather than metal ladders in 
water treatment and sewage plants. 



6.5 



Metal Appurtenances on 
Concrete Tanks 



6.5.1 Description 

A description of miscellaneous metal 
components in concrete tanks is given in 
paragraph 6.1.1 of this section. These 
metal components include main structural 
members, manhole covers, access ladders, 
safety lights and piping, and they are 
fixed to the concrete tank with metal 
fastenings. 




»i^-* 



Photo 6-11 Corroded steel manway cover 
and bolls. (WATFORD) 



6.5.2 Observations 

• In some cases, the main component 
may be in satisfactory condition but the 
fastenings, cast or drilled in the 
concrete, such as inserts, are corroded, 

often because they have not been 
protected satisfactorily against corrosion 
or because they are different to or are 
an inferior metal to the appurtenance. 
In the case of ladders and landings, this 
corrosion of fastenings can lead to 
unsafe conditions. 

• Working or critical parts of 
appurtenances such as hatch and manway 
hinges, threaded retaining bolts for 
manways, keyholes in locks, etc., are 
often badly corroded causing delays in 
entry for inspections and maintenance 
work, and costly replacement if threaded 
studs cannot be removed, for instance, 
(see Photo 6-11) 

• Steel mesh screens in cylindrical 
aluminum air vents installed to prevent 
entry of insects and birds into the tanks 
are almost invariably severely corroded 
because of the bi-metallic contact. 

6.6 Summary and Conclusions 

• The two most common metals used in 
concrete water tank construction are 
steel and aluminum. Steel is installed 
inside the concrete in the form of 
reinforcing bars and as post-tensioning 
tendons. Appurtenances on tanks such 
as external and internal ladders, 
landings, hatches, vents etc. are 
normally aluminum or galvanized steel. 

• Exposed metals on concrete tanks 
must be protected and maintained, in 
order to arrest or prevent deterioration. 

• Examples of severe corrosion of steel 
reinforcement are rare, but have 
occurred where the steel was exposed. 

• Hi-tech methods for the detection of 
corrosion "hot-spots", such as half cell 



6 - 12 



voltage measuring, should be validated 
by visual examination. The reason for 
this visual examination is because steel 
pipe jack-rods may be in close proximity 
to each other or touching in the wall. 
Severe corrosion activity in a jack-rod 
pipe, for example, may be read 
inaccurately as being in the prcstressing 
wires, which is erroneous. 

• Dense, high quality concrete provides 
good protection for steel reinforcement 
in concrete water tanks, providing the 
cover exceeds 25 mm. The cement 
content must be sufficiently high in 
order to maintain a pH value of 12 or 
more. This alkali environment protects 
the steel from corroding. 

• Corrosion of post-tensioning wires 
and strands will occur if the high tensile 
prestressing steel is unprotected by 
grease containing corrosion inhibitors 
(PTU tanks), by Portland cement grout 
injected into the ducts (PTB tanks), or 
by a minimum of 20 mm thickness of 
high cement content gunite cover coat 
(G tanks) and is exposed to the 
atmosphere and external or leakage 
water from the tank. 

• Stress-corrosion can cause brittle 
tensile failures in unbonded prestressing 
strands without rust signs, because the 
steel fails inside a plastic tube. 

• Detection of corrosion of 
post-tensioning tendons is sometimes 
difficult because they are hidden from 
view. Start inspections near leakage 
points. 

• Concrete tanks weakened by corroded 
internal or wirewound tendons may be 
strengthened by external 
post-tensioning. 

• Steel access tubes in concrete tanks 
may be primary structural elements and 
must be inspected and maintained in a 
corrosion free condition. 



• Severe pitting corrosion of aluminum 
has been observed in a concrete tank 
with a corroding steel access tube. 



• Many instances of the corrosion of 
tank appurtenances have been observed, 
especially fastenings and vital working 
parts, which need costly replacement 
when not maintained. 



6.7 



Recommendations 



• The metal parts of concrete tanks 
often form primary structural elements 
and must be inspected and maintained on 
a regular basis. 

• Post-tensioning tendons exposed to 
external water or tank leakage are 

vulnerable to weakening and failure due 
to corrosion and must be maintained and 
protected from this environment and all 
leakage or infiltration of water to 
prestressing steels must be stopped. 

• Aluminum ladders and fittings should 
not be used in chlorinated water, unless 
the metal is proven to be a corrosion 
resistant type alloy in that environment. 



6 - 13 



7.0 



CONCLUSIONS 



7.1 



Introduction 



7.2 



Concrete Deterioration 
Mechanisms 



The principal conclusions in this report 
relevant to the deterioration of concrete 
tanks, result from the five year study of 
the deterioration of 53 uninsulated 
concrete tanks in Ontario built since 
1956, but mainly since 1970. The study 
was initiated by the Ministry of the 
Environment in 1981 and includes the 
period up to the end of 1986. It 
incorporates findings from the applied 
research programme and references 
listed, as well as from investigations, 
inspections, repairs, and rehabilitation of 
the tanks. Eleven tanks have been, or 
will be replaced. The total 
rehabilitation programme in this five 
year period has cost over $15 million, 
including engineering, research, 
development, remedial 
repairs, waterproofing, replacement, 
insulation and cladding, freeze 
protection, construction contracts, and 
temporary storage. 

The study was started in 1981 because 
of the sudden failure of two new 
municipally owned concrete standpipes 45 
m high, the first in 1976 at 
DUNNVILLE, the second in 1980 at 
SOUTHHAMPTON and because of reports 
from various sources at that time of 
widespread deterioration and leakage of 
many of the other concrete tanks in 
Ontario with as little as 5 years service. 
This led to general concern for the 
condition of the tanks, their service life, 
and for the safety of the public. No 
above ground concrete water tanks have 
been constructed in Ontario since 1980. 



7.2.1 General 

A major factor in the extent of 
deterioration observed in above ground 
concrete water tanks in Ontario is the 
type of construction used, e.g. bonded or 
unbonded post-tensioning tendons, jack- 
rods left in the tank wall, inadequate 
gunite cover coat, cold joints produced 
during jump forming etc. This, 
combined with a cold region environment 
has resulted in catastrophic reductions 
in expected service life. 

7.2.2 Internal Ice 

Cold region environments can result in 
detrimental internal ice formations in 
uninsulated water tanks with low water 
turnover. Ice formations inside concrete 
tanks can cause significant hoop 
pressures on vertical walls subjected to 
rapid temperature rises. Pressures of 0.7 
MPa have been measured in ice caps due 
to expansion caused by a rise in 
temperature. These pressures are 
capable of splitting a concrete tank wall. 
Tanks with upward sloping walls reduce 
ice pressure effects. 



7.2.3 



Freeze-Thaw With Pressurized 
Water 



Freeze-thaw action on permeable 
concrete, saturated by water under 
pressure, can result in rapid failure of 
the concrete microstructure. Studies of 
this failure mechanism show that dilation 
or expansion of the water-filled micro 
pores occur on freezing at about -4°C. 
Part of this expansion on thawing has 
been demonstrated in this report to be 
non-elastic and irreversible. At the 
start of the thaw cycle, the ice may 
expand further, before melting. The 
greater expanded volume of the pores in 



7 - 1 



the microstructure may then be filled 
with additional water under pressure 
which on freezing causes a further 
expansion.- The accumulation of this 
ratchet effect of permanent residual 
dilation and associated incremental 
strains, results in the initiation of the 
failure of the matrix microstructure 
when the tensile strength of concrete is 
reached. 

7.2.4 Rate of Deterioration 

Observations of concrete deterioration 
indicate that the rate of deterioration in 
concrete water tanks is related primarily 
to the water pressure, freeze-thaw 
temperature amplitudes and frequencies, 
concrete permeability and the orientation 
of the wall section to the sun. The 
latter observation corroborates the 
postulation that it is temperature 
amplitudes and frequency which 
determine the rate and extent of 
deterioration. Additionally, temperature 
monitoring demonstrated that the 
amplitude and frequency of temperature 
changes were similar throughout the 
height of the tank. Since the 
deterioration was consistently at the 
lower portions of the tank this suggests 
that the rate of freezing in a single 
cycle is not as significant as previously 
considered. Dilation occurring in densely 
reinforced wall sections has resulted in 
progressive delaminations in reinforced 
concrete standpipes in cold regions. Air 
entrainment bubbles or voids, while 
reducing damage in other building 
structures, may contribute to damage 
under freeze-thaw conditions in high 
pressure water tanks. 

7.2.5 Freezing in \\all Voids 

Water under pressure, freezing in wall 
voids and cracks, can result in confined 
ice pressures measured as high as 
21 MPa producing spalling and 
delamination of concrete water retaining 
structures. Voids can occur under 
horizontal reinforcing, in ungrouted 



metal post-tensioning ducts, in unbonded 
tendons, and inside hollow coupled 
vertical jackrod pipes left in the wall 
after slipforming. Deterioration of the 
concrete is accelerated at leakage points. 

7.3 Expansion Joints 

Poor installation of plastic or rubber 
waterstops in expansion joints can result 
in leakage and costly repairs. 
Waterstops can act as dams in the wall 
to permeating water; concrete 
deterioration due to freeze-thaw action 
then often occurs on the interior face 
where the concrete is saturated by water 
under pressure. 

7.4 Corrosion of Prestressing Steel 

Losses of the protective gunite cover 
coat from wire wound prestressing wires 
has resulted in general corrosion and 
wire failure causing loss of 
circumferential prestress. This prestress 
loss results in vertical cracking of the 
tank wall. Poorly filled external 
post-tensioning anchorage recesses have 
permitted water to enter horizontal 
unbonded post- tensioning tendons 
resulting in stress-corrosion failure of 
strands. Metal ducts accidentally 
flattened during concrete placing has 
prevented the insertion of 
post-tensioning tendons. Lack of grout 
in ducts has allowed water to enter and 
freeze, causing cracking and 
delamination of the concrete wall, and 
general corrosion of the prestressing 
steel. 

7.5 Repair Methods 

7.5.1 General 

To be effective, the cause of the 
problems must be analyzed thoroughly 
before the repair is designed. Inspection 
and repair techniques expose people to 
the dangers of heights and high winds. 
Repairs are difficult, require specialist 
contractors and inspectors, and must be 



7 - 2 



carried out within Ontario's limited 
construction season. 

7.5.2 Bonded Waterproofing Coatings 

One method of preventing water freezing 
in the saturated permeable concrete and 
voids is to apply a bonded waterproofing 
coating on the inside of the concrete 
wall. The most effective coating 
materials used to date have been 100 
percent solids epo.xies which are non- 
toxic, tasteless and odourless. The 
epoxy coating has been applied in 3 
coats on a dry grit-blasted surface. 
Good bond (2Mpa minimum) is essential. 
Strict environmental control during 
epoxy application and curing is essential. 
A surfacing subcoat 3 mm thick must be 
used on rough concrete to avoid pinholes 
in the epoxy. Latex and epoxy mortars 
have proved to be good surfacing 
materials with adequate bond. It is 
difficult to completely prevent pinholes 
from forming in epoxy coatings without 
pressurizing the tank slightly during 
epoxy application. Water-filled blisters 
have been observed in epoxy coatings 
during the one year warranty 
inspections. These can be repaired 
satisfactorily, however in some cases the 
blisters were not repaired where it was 
considered they were stable. 
Any caulking used, must be durable 
under water pressure and resistant to 
chlorine during tank disinfection 
operations. 

7.5.3 Steel Liners 

Interior steel liners have been used 
successfully. Two methods of 
constructing steel liners inside concrete 
tanks have been used. One method 
employs bolts fastened to the concrete 
wall to provide temporary support of the 
steel plates, prior to welding. The other 
method is to construct a freestanding 
steel tank inside the concrete tank 
without using any mechanical 
connections to the concrete wall. A 
Portland cement grout having a pH 



greater than 11.5 provides protection of 
the steel against corrosion on the 
unpainted outside face. The steel liner 
face in contact with the water is 
protected against corrosion by applying a 
bonded coating. 

Where large roofs such as domes exist, 
access holes have been cut so that the 
steel liner plates can be loaded inside. 

In three elevated concrete water tanks, 
insulation has been placed between the 
steel liner and the inside of the existing 
wall, before grouting. External 
insulation and cladding has also been 
used on one other elevated tank and one 
standpipe where interior steel liners 
were installed. 

7.5.4 Plastic Liners 

A partial plastic liner has been installed 
as a cut-off over a defective wall floor 
expansion joint. The liner was a tough 
woven nylon fabric coated with PVC, 
and has performed satisfactorily for 5 
years. Recent inspection has revealed 
that the liner has now become somewhat 
stiffer and some holes have had to be 
repaired. 

Full height hypalon and polythene liners 
have been investigated, but to date have 
not been considered a practical or 
economical solution. 

7.5.5 External Post-Tensioning 

External post-tensioning has been 
carried out to control vertical hoop 
cracks in reinforced concrete standpipes, 
and to strengthen tanks initially 
post-tensioned by the wire winding 
process and where corrosion failure of 
the wires had taken place. Special 
anchorages were developed for single 
and twin strand tendons wound around 
circular tanks. Stage post-tensioning of 
the hoop tendons is necessary to control 
cracking of thin walls during the 
application of prestress. 



7 - 3 



On one project, water entering tubes has 
caused 3 percent tendon failure after 
seven years exposure to the elements. 
The failure from corrosion was 
apparently because of an inadequate 
original coating of protective grease on 
the strands. 



7.6 



Freeze Protection 



7.6.1 General 

Freeze protection systems have been 
developed and installed in concrete tanks 
in cold regions to attempt to eliminate 
internal ice formations, reduce the 
possibility of freeze-thaw cycling in the 
concrete, and to prevent freezing from 
taking place in the walls of standpipes 
constructed using unbonded tendons. 
Temperature sensors in the water and 
walls have been used to compare actual 
performance against heat loss 
predictions. 

7.6.2 Insulation and Cladding Systems 



7.6.3 Mixing and Heating Systems 

In extreme cases, external booster water 
heaters and internal eductor jet mixing 
units activated by external pumps are 
installed in the system. 

7.6.4 Air Gap Heating 

An air gap booster heating or guard ring 
system, external to the tank, has been 
developed to prevent concrete wall 
surface temperatures of existing tanks 
constructed with unbonded 
post-tensioning tendons, or where leaks 
and wall defects have been difficult to 
repair, from falling below freezing. 

7.7 Tank Types Not Recommended 

Two types of above ground water tank 
construction are not recommended in 
freezing environments. These are 
prestressed concrete tanks 
post-tensioned with unbonded tendons, 
and reinforced concrete standpipes. 



A system developed to insulate circular 
tanks consists of rigid Styrofoam SM 
sheets and corrugated pre-painted metal 
cladding to form a polygonal external 
wall separated from the concrete tank 
wall by an air gap. The system is 
removable so that regular exterior 
inspections of the concrete exterior may 
be performed. 

Measured performance of the external 
insulation system with an air gap shows 
a dampening down of local temperature 
fluctuations in the wall and a reduction 
factor of about 3 for the ratio of 
ambient to air gap temperature (e.g. at a 
temperature of -30°C the air gap 
temperature is -10°C). Differences 
between the theoretical temperatures 
determined from heat loss calculations 
and the actual recorded temperatures are 
attributed to air leakage in the system; 
however, harmful freeze-thaw cycling 
and internal ice formations have been 
eliminated. 



7 - 4 



8.0 



RECOMMENDATIONS 



8.1 



Introduction 



8.2.2 Interim Guidelines 



Although no above ground concrete 
water tanks have been constructed in 
Ontario since 1980, there may be reasons 
in the future for them to be construct- 
ed, such as the need for competition 
with steel because of the future high 
cost or shortage of steel. These recom- 
mendations are directed at describing 
how to try to build concrete water tanks 
in Ontario in the future, without under- 
going the problems experienced in the 
past. 



8.2 



Design & Construction of New 
Concrete Water Tanks 



8.2.1 Codes 

Codes and guidelines for the design and 
construction of above ground water 
tanks in Ontario and Canada generally 
do not specifically address the important 
freeze-thaw deterioration problems desc- 
ribed in this report and do not include 
specifications to ensure a satisfactory 
performance for the design service life 
of the structure in the Ontario environ- 
ment. Such codes should be developed. 
General guidelines, recommended to be 
followed where applicable, are as 
follows:- 

CAN3-A23.3-M84 

Design of Concrete Structures for 
Buildings. 

CSA S474 Part 4 

CSA Preliminary Standard for the 
Design, Construction and Installa- 
tion of Fixed Offshore Production 
Structures. 

ACI Committee 344 

Report on Recommendations for 
Design and Construction of Cir- 
cular Prestressed Concrete Tanks. 



Interim guideline recommendations of 
minimum requirements for the design and 
construction of new above ground water 
tanks in Ontario (prepared by W. M. 
Slater & Associates Inc.) have been 
included as Appendix 'A' in this report. 

The main requirements in the guidelines 
are as follows:- 

• Design and construct for a service 
life of 50 years with minimum main- 
tenance. 

• Concrete permeability shall not 
exceed 0.25 x 10"^^ m/s, as measured 
first by test mixes and confirmed by 
cores taken from the actual tank. 

• Concrete tanks shall be leakproofed 
by the installation of a liner or the 
interior shall be coated with an 
approved waterproof coating. A one year 
extended warranty against future leakage 
shall be required after a satisfactory 
leakage test. 

• Concrete water tanks shall be 
biaxially post-tensioned with external or 
grouted tendons to give a reserve comp- 
ression of 1.5 MPa in horizontal and 
vertical directions after consideration of 
all applied and environmental loads. 

• Concrete water tank structures, and 
the stored water, shall be protected 
from freezing by insulation, air gap 
heating, or other methods. No internal 
ice formations will be permitted. Carry 
out heat loss calculations and assess the 
requirement for either a passive or an 
active ice prevention system. 



- 1 



8.3 



Maximum Head 



The maximum head of new concrete 
ground or elevated tanks should be 
12 m. No standpipes shall be 
constructed. 

8.4 Technology Transfer 

The principal conclusions and recomm- 
endations in the tank applied research 
reports should be made available to the 
engineering profession, consultants, and 
operations personnel, by means of 
reports, seminars, papers, etc. so that 
repairs underway, and future concrete 
tank design and construction, and 
maintenance, might benefit from the 
work. 

8.5 Durability 

Further development is required in des- 
ign, construction, and materials to opt- 
imize the economy of a new breed of 
insulated concrete tanks in the following 
areas: 

• concrete mix designs and methodology 
to result in minimum construction 
defects and a minimum coefficient of 
permeability. 

• design for better concrete placement 

• improved wall/floor joint details 

• development of a thin (3 mm thick) 
stainless steel liner (no maintenance) 
waterproofing system for concrete tanks 
using normal concrete mixes. 

8.6 Further Applied Research 

• The models presented in chapter 4 of 
this report should be verified under 
laboratory conditions. Development of a 
laboratory model will enable greater 
understanding of the rate of concrete 
deterioration under varying hydrostatic 
pressures and saturated freeze-thaw 
conditions. It will also permit 



preventative measures such as concrete 
permeability, liners, coatings, penetrating 
sealers and imperfections in these types 
of barriers to be evaluated accurately. 

• The potential to passively maintain 
the tank inlet water at temperatures 
greater than 2°C should be investigated. 
Possible sources of heat gain would be 
the source water and the feeder pipe 
system. Seasonal fluctuations at these 
locations should be monitored to opt- 
imize system design and minimize the 
energy costs accrued during the service 
life of the tank. 

• Cost effective and less disruptive 
methods of internal inspection of water 
tanks should be developed in order 

to allow regular inspection and minimize 
maintenance costs. Remotely operated 
underwater vehicles equipped with video 
cameras may allow inspections to be 
carried out without having to empty 
tanks and disinfect after the inspection 
has been completed. 

• Early warning of any undesirable or 
potentially dangerous conditions should 

be made available to operators. Where 
possible these should be remotely sensed 
and controlled to avoid the necessity of 
manual inspection during severe winter 

conditions. Currently this may require 
the operator to climb 45 m on external 

ladders at temperatures below -25°C. 
Continuous monitoring of water inlet and 

outlet, concrete wall and the 
temperature of other significant loca- 
tions as well as viewing internal ice and 
wall coating conditions etc. could be 
made available in the water plant by 
installing equipment developed from 
monitoring technology already used in 

the tank rehabilitation programme. 



- 2 



9.0 



LIST OF REFERENCES 



1) Slater, VV.M., & Associates Inc. 
Interim Report on Ontario Concrete 
Water Retaining Structures". 
Ministry of Environment of Ontario, 
Project WTS-001, 1982 

2) Slater W.M., "Concrete Water Tanks 
in Ontario". Proceedings, Canadian 
Society of Civil Engineering 1983 
Annual Conference, June 1-3, 1983 

3) Ministry of Transportation and 
Communications of Ontario. 
"Bridge Deck Rehabilitation, Manual", 
Part One, Downsview 1983. 

4) Helmuth, R.A., "Capillary Size 
Restriction on Ice Formation in 
Hardened Cement Pastes", 
Proceedings, Fourth International 
Symposium on the Chemistry of 
Cement, Washington, D.C., V.2, 1960. 

5) Fagerland, G., "The Significance of 
Critical Degrees of Saturation at 
Freezing of Porous Brittle 
Materials". ACI Publication SP-47, 
American Concrete Institute, Detroit, 
1975. 

6) Beaudoin, J.J. and Maclnnis, C, 
"Dimensional Changes of Hydrated 
Portland Cement Mortar Due to Slow 
Cooling and Warming". ACI 
Publication SP-47, American 
Concrete Institute, Detroit, 1975. 

7) Kays, W.B., "Construction of Liners 
for Reservoirs, Tanks, and Pollution 
Control Facilities", John Wiley & 
Sons, 1977 

8) Gray, W.S., "Reinforced Concrete 
Water Towers, Silos, and Gantries". 
Concrete Publications Limited, 1953 



9) Hazlehurst, J.N., "Towers and 
Tanks for Water Works", John Wiley 
& Sons, 1910 

10) Creasy, L.R., "Prestressed Concrete 
Cylindrical Tanks", John Wiley & 
Sons, 1961 

11) Manning, G.P., "Concrete Reservoirs 
and Tanks", Concrete 
Publications Limited, 1967 

12) Gray, W.S., "Reinforced Concrete 
Reservoirs and Tanks", Concrete 
Publications Limited, 1954 

13) Powers, T.C., "Freezing Effects in 
Concrete" ACI Publication SP-47 
American Concrete Institute, 
Detroit, 1975. 

14) Verbeck, G., "Significance of Tests 
and Properties of Concrete and 
Concrete - Making Materials" 
Chapter 18. ASTM Publications STP 
169B, 1978. 

15) Carrier, R.E. and Cady, P.D., 
"Moisture Distribution in Concrete 
Bridge Decks and Pavements" ACI 
Publication SP-47, American 
Concrete Institute, Detroit, 1975. 

16) Mills, R.H., "The Permeability of 
Concrete for Reactor Containment 
Vessels", Report 84-01 University 
of Toronto, July, 1983. 

17) Powers, T.C., "A Working 
Hypothesis for Further Studies of 
Frost Resistance of Concrete". ACI 
Journal, Proceedings, V. 41, No. 3, 
February 1945. 



18) "Frost Durability of Clay Bricks - 
Evaluation Criteria and Quality 
Control" - Proceedings No. 8 
CBAC/DBR 1984 

19) Neville, A.M., "Properties of 
Concrete" Pitman Publishing 
Company, 1974. 

20) Maclnnis, C. and Whiting, J.D., "The 
Frost Resistance of Concrete Subject 
to a De-icing Agent", Cement and 
Concrete Research, Vol.9, 1979. 

21) Neville, A.M., "Creep of Concrete: 
Plain, Reinforced and Prestressed", 
North-Holland Publishing Co., 
Amsterdam, 1970. 

22) Khalil, S.M., Ward, M.A., and 
Morgan, D.R., "Freeze-Thaw 
Durability of Non Air-Entrained High 
Strength Concretes Containing 
Superplasticizers", Proceedings, First 
International Conference on 
Durability of Building Materials and 
Components, Ottawa, Canada, 
August, 1978. 

23) Marusin, S., "The Effect of 
Variation in Pore Structure on the 
Frost Resistance of Concrete, 
Cement and Concrete Research, 
Vol. 11, 1981. 

24) Litvan, G.G., Maclnnis, C, and 
Grattan-Bellow, P.O., "Cooperative 
Test Program for Precast Concrete 
Paving Elements", Proceedings, First 
International Conference on 
Durability of Building Materials and 
Components, Ottawa, Canada, 
August, 1978. 



25) Adkins, D.F. Laboratory Duplication 
of Surface Scaling. Concrete 
International Vol 8 No. 2 
February, 1986 



26) National Building Code of Canada, 
1985. 

27) Wood, J.H., Adams, J.R. 
"Temperature Gradients in a 
Cylindrical Concrete Reservoir", 
Proceedings of the 6th Australasian 
Conference on the Mechanics of 
Structures and Materials, 
Christchurch, N.Z. 1977. 

28) Tuomioja, M., Jumppanen, P. and 
Rechardt, T., "Jaan Lujuudcsta ja 
Muodonmuutoksista" (The Strength 
and Deformation of Ice), 
Rakennustekniikka, 1973:1 pp 
233-246. 

29) Bergdahl, L., "Thermal Ice Pressure 
in Lake Ice Covers", Report A2, 
Department of Hydraulics, Chalmers 
University of Technology, Sweden, 
1978. 

30) Reynolds, CE., "Reinforced 
Concrete Designer's Handbook", 
Concrete Publications Ltd., London, 
1961. 

31) Ghali, A., "Circular Storage Tanks 
and Silos", Spon Ltd., London, 1979. 

32) Portland Cement Association, 
"Circular Concrete Tanks without 
Prestressing". 

33) Markus, G.Y., "The Theory and 
Computation of Circular 
Symmetrical Structures". 



34) Hibbitt, Karlsson and Sorensen, 
Inc., "ABAQUS - Structural and 
Heat Transfer Analysis Program" 

35) Schupack, M., 1982. Protecting 
Post-tensioning Tendons in 
Concrete Structures, Civil 
Engineering - ASCE. 



9 - 2 



10.0 



GENERAL REFERENCES 



ACI Committee 344. 1970. Design and 
construction of circular prestresscd ' 
concrete structures. ACI Journal, 
September, pp/ 657-672. 

ACI Monograph No. 3, 1966, "Freezing 
and Thawing of Concrete - Mechanisms 
and Control", Detroit. 

Aldworth, G., Staton, R., (MacLaren 
Engineers), and Slater, W.M., (W.M. 
Slater & Associates Inc.). Preliminary, 
(1985). "Freeze Protection for Above 
Ground Concrete Water Tanks in Cold 
Regions". 

ASCE Civil Engineering. 1981. Structural 
failures. New York, December, p. 44. 

Beeby, A.W. 1978. Cracking: what are 
cracking limits for? Concrete, London, 
July, p. 3. 

Campbell, T.I., and Kong, W.L., (Queen's 
University). April, (1986). "Ice Loading 
in Elevated Water Tanks". 

Carpenter, C.H. 1982. Constructing and 
maintaining distribution storage 
structures. AWWA Journal, Vol. 74, No. 
11, Denver, Col., pp. 581-583. 

Cedolin, L., Poli, D., loro, I. 1983. 
Experimental determination of the 
stress-strain curve and fracture zone for 
concrete in tension. Proceedings of the 
International Conference on Constitutive 
Laws for Engineering Materials, 
University of Arizona, Tucson, Arizona, 
pp. 393-398. 

Code of practice for concrete structures 
for the storage of liquids. 1978. Part I, 
design based on "resistance to cracking" 
approach, Wellington, N.Z. 



Crowley, F.X. 1976. Maintenance 
problems and solutions for prestresscd 
water tanks. AWWA Journal, Denver, 
Co., November, pp. 579-585. 

Frederking, R., Sinha, N.K. 1977. Ice 
action on wharf at Strathcona Sound. 
Proceedings of Fourth International 
Conference on Port and Ocean 
Engineering under Arctic Conditions, 
Memorial University of Newfoundland, 
St. John's, pp. 707-717. 

Gold, L.W. 1958. Some observations on 
the dependence of strain on stress for 
ice. Canadian Journal of Physics, Vol. 
36, Ottawa, Ontario, pp. 1265-1275. 

Gold, L.W. 1960. The cracking activity 
in ice during creep. Canadian Journal of 
Physics, Vol. 38, No. 9, Ottawa, 
Ontario, pp. 1137-1 148. 

Gold, L.W. 1965. The initial creep of 
columnar-grained ice. Part I: Observed 
behaviour. Part II: Analysis. Canadian 
Journal of Physics, Vol. 43, Ottawa, 
Ontario, pp. 1414-1422 and 1423-1434. 

Gold, L.W. 1966. Observations on the 
movement of ice at a bridge pier. 
Proceedings of a Conference on Ice 
Pressures against Structures at Laval 
University, Quebec City, pp. 135-141. 

Gold, L.W. 1966. Elastic and strength 
properties of fresh-water ice. 
Proceedings of the Conference on Ice 
Pressures against Structures at Laval 
University, Quebec City, pp.13-23. 

Grieve, R. (Golder Associates). May, 
(1984) and February, (1986). 
"Temperature Monitoring, Ontario 
Concrete Water Tanks". 



10 - 1 



Harper, W.B. 1982. Inspection, painting, 
and maintaining steel water tanks. 
AWWA Journal, Vol. 74, No. 11, Denver, 
Col., pp. 585-587. 

Hassett, R.W. 1980. Design and 
construction of water storage facilities. 
Strand Associates, Inc., Madison, 
Wisconsin. 

Hertzberg, L.B., Westerback, A.E. 1976. 
Maintenance problems with wire-wound 
prestressed concrete tanks. AWWA 
Journal, Denver, Col., December, p.44. 

Krajcinovic, D., Selvaraj, S. 1983. 
Constitutive equations for concrete. 
Proceedings of the International 
Conference on Constitutive Laws for 
Engineering Materials, University of 
Arizona, Tucson, Arizona, pp. 399-406. 

Krausz, A.S. 1963. The creep of ice in 
bending. Canadian Journal of Physics, 
Vol. 41, No. 1, Ottawa, Ontario, pp. 

167-177. 

Krausz, A.S. 1966. Plastic deformation of 
fresh-water ice. Proceedings of the 
Conference on Ice Pressures against 
Structures at Laval University, Quebec 
City, pp. 5-12. 

Krausz, A.S. 1981. Adfreeze strength of 
model piles in ice. Canadian Geotechnical 
Journal, Vol. 18, No. 1, pp. 8-16. 

Kronen, H., Anderson, J.H. 1982. 
Concrete exposed to cryogenic 
temperature, Nordisk betong, Norway, pp. 
13-17. 



Larrabee, R.D., Billington, D.P., Abel, 
J.F. 1974. Thermal loading of thin-shell 
concrete cooling towers. ASCE Journal, 
Structural Division, December. 

Mackenzie, K., (Dalhousie Materials), 
Slater, W.M., (W.M. Slater & Associates 
Inc.) and McGrenere, P., (Knox Martin 
Kretch) (Editing). Preliminary, (1985). 
"Evaluation of Waterproof Coatings for 
Concrete Water Tanks". 

Perkins, P.H. 1978. Concrete structures: 
repair, waterproofing and protection. 
Reprinted. Applied Science Publishers 
Ltd., Barking, Essex, England, 302 p. 

Pitkanen, A. c. 1980. Roihuvuori water 
tower, Helsinki, Finland. Publication 
unidentified, 2 p. 

Priestley, M.J.N. 1976. Ambient thermal 
stresses in circular prestressed concrete 
tanks, ACI Journal, October. 

RILEM (Reunion internationale des 
laboratoires d'essais et de recherches 
sur les matériaux et les constructions). 
1981. Materials and structures. 
Properties of set concrete at early 
ages-state-of-the-art report. Vol. 14, 
No. 84, Paris, France, pp. 399-449. 

Schupack, M. 1981. How to inspect and 
evaluate prestressed concrete tanks. 
Public Works, September, pp. 88-89. 

Schupack, M., Suarez, M. 1981. Some 
recent corrosion embrittlement failures 
of prestressing systems in the United 
States. ACI Journal, Vol. 27, No. 2, 
Chicago, Illinois, pp. 38-56. 



10 - 2 



Slater, W.M. 1975. Stage Timoshcnko, S., Woinowsky-Krieger, S. 

post-tensioning-a versatile and economic 1959. Theory of plates and shells, 

construction technique. PCI Journal, Vol. McGraw-Hill. 

20, No. 1, Chicago, Illinois, pp. 14-27. 

Vessey, J.V., Preston, R.L. 1978. A 
Slater, W.M. & Associates Inc. 1982. critical review of code requirements for 

Study of concrete elevated water tanks. circular prestressed concrete reservoirs. 

Part 1, Analysis and evaluation of FIP 8th Congress, London, 

defects using a numerical coding system. 
Part 2, Interim guideline 
recommendations. Report 3 (for review). 
Project WTS-001, Toronto Ontario, 
111 p. 



10 - 3 



11.0 



APPENDIX A 



INTERIM GUIDELINE RECOMMENDATIONS OF MINIMUM REQUIREMENTS 
FOR THE DESIGN AND CONSTRUCTION OF NEW ABOVE GROUND CONCRETE 
WATER TANKS IN ONTARIO 



11.1 Introduction 

This report has concluded that under 
high pressure, the permeability of normal 
structural concrete is insufficient by 
itself, to eliminate the deterioration of 
the walls of a concrete water tank 
indefinitely, in the environmental condi- 
tions prevailing in Ontario. It is, 
therefore, essential that either the 
concrete is prevented from attaining a 
high degree of saturation, or that cyclic 
freezing and thawing is eliminated. 

Based on the experience gained to date, 
reduction of severe freezing and thawing 
by means of external insulation is the 
more positive protection. It is less 
dependent on the site and seasonal 
conditions, and is less costly than the 
application of waterproof coatings and 
steel liners. A second, but important 
benefit is the elimination of the internal 
formation of ice round the tank walls. 
In addition, small deviations from 
specification do not have radical 
consequences to the expected life of the 
tank. Since the insulation is external to 
the tank structure, it can be rectified 
without undue interruption to normal 
operations. Conversely, protecting a 
tank by means of an internal coating 
only, is less assured, since it is difficult 
to install, may have a limited life, and 
will not prevent the internal formation 
of ice. Internal repairs result in costly 
delays to normal operations. 

The following interim guidelines will 
result in concrete water tanks which 
have higher initial, but reduced 
maintenance costs than have been 



experienced. It is the intention of the 
guidelines to recommend standards 
which will ensure the construction of a 
tank which will not deteriorate during 
its expected service life of fifty (50) 
years (minimum), and should not require 
costly maintenance. It is recommended, 
therefore, that alternate designs should 
be compared on the basis of initial and 
maintenance costs. 

11.2 Scope 

These recommendations are not 
"stand-alone" and are limited to the 
primary design and construction 
requirements of new concrete water 
retaining structures only, and exclude 
details of appurtenances and the 
system, except where noted. 

11.3 References 

The recommendations are additional to 
and take precedence over the 
requirements of the following listed 
codes, standards, and references: 

• Province of Ontario 
Building Code. O. Reg. 419/86 
August 6, 1986. 

• The National Building Code of 
Canada (1985). 

• Supplement of NBC (1985). 

• CSA Standard CAN3-A23.1-M77, 
CAN3-A23.2-M77 

Concrete Materials and Methods of 
Concrete Construction 
Methods of Test for Concrete. 



11 - 1 



CSA Standard CAN3-A23.3-M84. 
Design of Concrete Structures 
for Buildings. 

ACI Standard 318-83 Building Code 
Requirements for Reinforced 
Concrete. 

ACI Committee 344 Report on the 
Design and Construction of Circular 
Prestressed Concrete Structures 
(1970). 

ACI Committee 350 Report on 
Concrete Sanitary Engineering 
Structures (1977). 

AWWA Standard for Welded Steel 
Tanks for Water Storage 
ANSI/AWWA DlOO-79. 

Ministry of Environment Standard 
Specifications. 



11.4 Design 

11.4.1 Design Philosophy 

In addition to the strength requirements 
of the specified codes, standards and 
references listed in section 11.1.3 for 
Applied Loads, design for the following 
conditions shall be carried out: 

• Environmental loads (ice, thermal 
differential, etc.) 

• Durability for the actual freeze-thaw 
environment. 

• Provide a service life goal with 
minimum maintenance, of fifty (50) years 
(minimum). 

11.4.2 Tank Design Requirements 

.1 Insulation 

Concrete tanks shall be protected from 
freezing and shall be insulated and clad 
on the exterior to reduce heat loss and 



prevent significant internal ice 
formation. 

.2 Ice Prevention 

• An ice protection system shall be 
designed to prevent the tank water 
from freezing during the worst winter 

conditions anticipated during the 
service life of the structure (50 years). 

• Actual tank heat loss is dependent 
on the quality of the construction and 

air tightness of the insulation, 
therefore, heat loss calculations shall 
be made assuming a minimum tank 
water temperature of 2°C, unless more 

accurate data is available and based 
upon temperature monitoring results of 
similar systems and materials. 

.3 Inspection 

The cladding and insulation shall be 
demountable to allow exterior 
inspections and maintenance. 

.4 Mixing/Heating 

• Mixing and booster heating of the 
water in some tanks with low turnovers 
may be necessary to supplement the 
heat loss reduction from the insulation 
for short periods during severe winters. 

• The provision of heated air in the 
air gap between the insulation and the 
concrete is an alternative to the above 

where it is important that the concrete 
surface temperature does not fall below 
0°C. 

.5 Prestressed Construction 

All concrete water tanks shall be of 
prestressed construction. 

.6 Prestress Design 

Concrete water tanks shall be 
post-tensioned in the vertical and 
horizontal directions with the following 



11-2 



reserve compression after service loads 
are considered:- 

o Reserve Compression (Hoop) 

The minimum final (after losses) reserve 
hoop (circumferential) prestrcss shall be 
1.5 MPa (214 psi). 

• Reserve Compression (Vertical) 

The minimum final (after losses) reserve 
vertical prestress shall be .5 MPa (71 
psi). 

• Post-tensioning 

Post-tensioning shall be carried out 
using internally bonded (grouted) 
tendons, or external tendons and 
hardware permanently protected against 
corrosion. 

.7 Concrete Wall Thickness 

The minimum thickness of the concrete 
wall shall be 200 mm (8 in.). 

.8 Waterproofing 

Concrete water tanks shall be maintained 
permanently leaktight using impermeable 
concrete, bonded waterproof coatings, 
steel liners or other liners of 
impermeable material proven to be 
durable in chlorinated water. 

.9 Coatings 

• Waterproof coatings shall be 100 per 
cent solids epoxies or a modified 
cementitious slurry with fabric and shall 
be applied to all concrete in contact 
with water. 

Epoxy coatings shall be applied on dry 
concrete in a minimum of 3 coats (each 
coat 6 mils minimum) on top of 
surfacing mortar when the original 
surface exceeds a critical roughness or 
exhibits too many surface pin holes. 



• The permeability of the concrete for 
walls to be waterproofed with bonded 
coatings shall not exceed 2.5 x 10"^^ 

m/s tested at 90 days. 

.10 Liners 

• Where plain or stainless steel is 
used as an internal waterproof liner it 

shall be 5 mm (3,16 inch) minimum 
thickness. Where plain steel is used, it 
shall be protected on the face in 
contact with the water by an MOE 
approved single component epoxy or 
vinyl paint system. 

• The space between the liner and the 
concrete wall shall be filled with an 
MOE pre-approved cementitious grout 
having a minimum pH value (hydrogen 
ion value) of 12. 

11.5 Construction 

11.5.1 Concrete Quality 

• The concrete mix, wall design, and 
construction shall result in a crack free 
and leaktight wall. No cold joints will 
be permitted without waterstops being 
installed. 

11.5.2 Slip Forming 

• Jack-rod pipes used for jacking up 
the forms shall be removed and the 
wall void formed by the pipes grouted 
up from the bottom to the top, or the 
pipe sections shall be completely 

grouted individually from bottom to top. 

• Locations of the jack-rods shall be 
recorded on as-built shop drawings. 

• Details of grout vents and methods 
of grouting jack-rods shall be approved 
by the authority before construction of 
the tank. 

• Horizontal micro-cracking caused by 
binding of the slipform shall be 
prevented, or if they occur they shall 



11 - 3 



be repaired by routing out and tilling 
with epoxy mortar. 

11.5.3 Jump Forming 

• Pour iieights shall not exceed 
2.5m (8 ft. 2 inches). 

• Form vibrators clamped to the forms 
shall be used in addition to internal 
vibrators. 

• Waterstops shall be installed at the 
top (and bottom) of each lift. 

11.5.4 Vertical Waterstops 

• Only plain, de-greased, mild steel 
waterstops shall be used. 

• Minimum dimensions shall be 200 mm 
(8 ins.) for the base and 150 mm 

(6 ins.) high elsewhere, and 6 mm 
(1/4 in.) thiclt. 

• Vertical joints in steel waterstops 
shall be continuously lap welded (fillet). 

11.5.5 Concrete Joint Preparation 

• Construction joints shall be 
sandblasted to remove all laitance, 
before placing fresh concrete. 

11.5.6 Concrete Curing 



11.6.2 Final Inspection of Structure 

• An external maintenance inspection 
to prove that the tank is leaktight and 
sound, and an internal inspection to 

prove that the waterproof coating or 
liner is satisfactory, shall be made 
after the tank has been in continuous 

service for a period of between 9 and 
12 months. 

• Any deficiencies shall be repaired 
and the tank re-inspected after a 

further equal period in 
service. 

11.6.3 Heat Loss and Ice Prevention 

• External ambient temperatures, 
temperatures in the air gap between 
the insulation and the wall, and the 
tank water temperatures shall be 
measured during an extreme cold period 
of the first winter of tank operation. 

• Verify that the insulation is airtight, 
and that the freeze protection system 

is functioning satisfactorily, according 
to the design and heat loss 
calculations. 

• Sensors installed in the locations 
described above are recommended for 
temperature monitoring and control of 
water temperature. 



• Continuous wet curing and covering 
of the concrete with burlap or tarpaulins 
shall be carried out for a minimum of 7 
days after stripping the forms. 

11.6 Quality Assurance And Tank 
Performance Testing 

11.6.1 Leakage Testing 

• A 3 day leakage test shall be carried 
out and any leaks or damp spots 
repaired and retested, if necessary, 
before applying waterproof coatings or 
exterior insulation. 



• Inspect top surface of water at wall 
in late winter to observe that 
significant ice formation(s) does not 
exist. 

11.7 Security And Safety 

11.7.1 Security Fence 

• Tanks should be enclosed within a 
perimeter security fence to prevent 
vandalism to the cladding, and 
insulation (fire), climbing by 
unauthorized persons, etc., and to 
provide protection from falling objects. 



II 



ice, etc., and during maintenance or 
repair operations. 

11.7.2 Safety 



• New concrete water tanks should be 
located at a minimum distance of twice 
(x 2) their height from public 
thoroughfares or at least three times (.x 
3) their height from occupied buildings. 
This will increase the safety of the 
public during demolition or during a 
major disaster causing collapse or 
toppling of the tank. 



11.8 Miscellaneous .And .Appurtenances 

• Aluminum. Permanent aluminum 
ladders shall not be used in contact 
with chlorinated water. Use 
galvanized steel or fibre glass. 



• Inlet/Outlet Pipes. Separate inlet 
and outlet pipes to provide some 
natural mixing of the tank water. 



12.0 APPENDIX B 

DILATION EXPANSION OF SPHERICAL REGION WITHIN A LARGER SPHERE 



A confined saturated spot dilating under 
freezing will result in a considerable 
internal pressure which can cause 
fracture of concrete. The magnitude of 
resulting stresses will strongly depend 
on geometrical dimensions and the shape 
of a saturated zone. A variety of 
qualitative effects and magnitudes of 
stresses caused by expansion of confined 
zones can be investigated by considering 
expansion of a spherical region confined 
within a sphere of a larger diameter. 
Due to the symmetry of the problem, an 
analytical solution for stresses and 
deformations is possible. 

It is physically clear that the expansion 
of an inner sphere within a larger 
sphere will result in a compressive 
lateral stress accompanied by tensile 
hoop stresses. Maximum tensile stress 
will be at the interface of the dilating 
and non-dilating zones. If the tensile 
stress is large enough, brittle fracture 
will be initiated in concrete. Further 
increases in the dilation will cause 
fracture propagation. Due to the 
symmetry of the problem, the fractured 
zone can be assumed to be spherical. In 
general, there could be three different 
zones (Figure 12.1): 

• Dilating zone - radial and hoop 
compression (zone A) 

• Fracture zone - radial compression, 
zero hoop stress (zone B) 

• Elastic zone - radial compression, 
hoop tension (zone C) 



Stresses and displacements in each of 
the above zones can be determined by 
solving equations of static equilibrium 
and geometrical compatibility. The 
location of the interface between 
fractured and elastic zones can be 
found from the condition that the 
tensile strain in the elastic zone docs 
exceed the limiting tensile strain at 
fracture. Figure 12.1 illustrates various 
features of the solution. 

Pressure at the interface of the dilating 
and non-dilating zones is proportional 
to dilatant strain and increases with 
the thickness of the non-dilating 
material covering the dilating region 
(Figure 12.1a). 

Fracture at the interface is initiated at 
a certain dilatancy (Figure 12.1b). The 
more confined the dilating zone is, the 
larger the dilatancy required to initiate 
fracture. 

Further increase in dilatancy causes 
fracture propagation. When dilatancy 
reaches a certain "critical" value, the 
process of fracture propagation becomes 
unstable and the entire sphere is 
ruptured (Figure 12. Ic). 

The dilatant strain which causes the 
rupture increases with the thickness of 
non-dilating material (Figure 12. Id). 



12 - 1 




A- RADIUS OF DILATING ZONE 
B - RADIUS OF ELASTIC ZONE 
C- RADIUS OF FRACTURED ZONE 
Ed-LINEAR STRAIN DUE TO DILATANCY 

et-LiMrriNG tensile strain 

Ec- YOUNG'S MODULUS 

P- PRESSURE AT INTERFACE OF DILATING 
AND FRACTURED ZONES 





r 


09 




/" PRESSURE 


08 
7 


- / B£FOF?E INITIATION 
( OF FRACTURE 


-^ 0.6 
EcEd 

05 


■ Cé^' 


Û4 




03 


. ' \ — ^ 


02 




01 

o 


1 1 1 1 1 I J 



15 
B/A 






DILATANT STRAIN 
TO INITIATE 
FRACTURE 




ed 

ei 



PROFOGATON 


B/A = 8 


OF FRACTURE 


ZONE 


/ ^,^ UNSTABLE 


/ 


INSTABLE "^ 


/. 


B/A = 6 





Figure 12.1 Rupiuve of a brittle sphere due to dilation 



12 - 2 



13.0 



APPENDIX C 
TERMS OF REFERENCE 



The terms of reference included as Appendix 'A' in the agreement between the 
Ministry of the Environment and W.M. Slater & Associates Inc. for "The 
Investigation of Elevated Concrete Water Retaining Structures" dated June 24, 1981 
were as follows: 



1) Inspect approximately 30 concrete 
water retaining structures in all six 
MOB Regions to establish their 
condition, define deficiencies, 
recommend remedial measures and 
develop standard methods for repairs 
where warranted. 

2) Study available material on record for 
each structure inspected and relate 
the findings to the condition of the 
structure at the time of inspection. 

3) Prepare a report on each structure 
investigated and inspected including 
the method of effecting permanent 
repairs together with associated 
costs. 

4) Make necessary arrangements for 
local co-ordination of investigations 
including testing of materials and any 
other analyses. 



5) Prepare design and maintenance 
guidelines including standard 
methods of repairs and inspections. 

6) Record all findings and submit a 
"General Study Report"in a format 
suitable for general distribution. 

7) Identify structures requiring urgent 
repairs before the onset of winter 
1981 and recommend courses of 
action to protect such structures. 

8) If required, prepare specifications 
for tendering repairs and perform 
the supervision of construction 
related thereto. 

9) Act as expert witness for the 
Crown, if required. 



Explanatory Note 

In response to items 4, 5, 6, 7 and 8 in the above terms of reference, the report on 
"Immediate Research Needs" dated October 20, 1982 was prepared. Funds were 
subsequently made available to carry out applied research in the areas of structural 
effects of freezing, ice (freezing) prevention, and leakproof ing. This report records 
the observations made and the applied research carried out in the area of freeze- 
thaw failure mechanism. It was later decided to expand the terms of reference to 
include sections on the deterioration of metals, and the repair of concrete tanks. 



Provincially owned at the time of 
construction 



13 - 1
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